I gave my friend this puzzle to solve, but for some reason he couldn't get anywhere with it. The instructions were really simple too: "In eleven cells, the across and down entries lightly clash with one another."
Maybe you can find out what knowledge he was lacking. When read row by row, the special cells reveal what my friend would have needed in order to be able to solve the puzzle. When read column by column, the same cells reveal more specifically what type the thing he needs is.
Across
1. Question yours truly with an unknown value in series of questions (4)
5. A term of endearment described by "term of endearment"? That's dull! (5)
9. Prophet's wife, missing husband, returns where the Holy Land is (4)
12. Reportedly what people from Oxford and Cambridge did on the Thames Travel route (4)
13. Hint for cooking: Cup to be dipped in quickly (7)
14. The writer contacted retired couple (4)
15. In Czechia, George II is behind judge having queen arrested (4)
16. Basis for Marvel hero to ignore gravity (4)
17. Never stopping by a port city on the Black Sea (9)
20. What a bull catching up to a careless runner might do is... turn about? (4)
24. Transported electron with carbon? A type of electricity (8)
25. Visiting one taking part in Pride is a queer affair (7)
30. Frenchmen who wait for newspaper to back Conservatives? (7)
32. Instances of cutting while McCartney played with them (7)
34. Twisted clothes long sorted incorrectly (7)
35. Champagne's most prominent figure to harbour hostility towards auxiliary winemakers (7)
39. On the radio, short bit of code that's turned out, ultimately, pretty neat (4)
41. Criminal pinching unlimited funds, twenty large (9)
45. Disregarding court order? That's drastic! (4)
46. Cancel visit (4)
48. Jay regularly visited Monaco to see Gilberto? (4)
49. Jack and I separately participate in cacao production somewhere in Corsica (7)
50. A title one might spot in French opera programmes – even ones in Paris, dear! (4)
51. Practice negotiating on vacation to get dough in 9 across (4)
52. Woke singer who filled in for B. Johnson on tour (5)
53. Alone in French city, cycling (4)
Down
1. Intelligent medical practitioner, with no formal training, nurses me to avoid onset of anaphylaxis (5)
2. Doing as much as qualified (2,2)
3. Comedy's follow-up to Me, Myself & I by Ms Russo (5)
4. Jewish leader supports God's reign (3)
6. One-third shorter tropical island native (4)
7. What Romans wore when they wanted to visit the Forum to chat boastfully (5)
8. Lament being backed with no currency (4)
10. For one, Sean Connery has old home with grand entrance that's dashing (7)
11. A new head for toy soldier? (3)
13. Friends maybe hold meeting, coming halfway (6)
18. Boxer and setter's offspring that's scratched male dogs back in forest (5)
19. A nose blown for a long time (5)
20. Wide open crack filled with spare part? (6)
21. Green Book writer (3)
22. Surname of Spanish origin spelt out by old Hopi trails near Arizona (4)
23. Partially Germanic etymology for word meaning "a subtle point" (6)
26. Pleasantly rustic, rad houses Charlie spotted to the south from Austria's capital (8)
27. One hundred and four criminal Greek people live here (5)
28. Fights come up over conclusion of deposition given under oath (5)
29. Unwavering loyalty? Vacuous and unattractive! (4)
31. Plaster company supports belt-tightening efforts, in retrospect (6)
33. Cooler time to perform (3)
36. Bubbly before red results in buzzing (5)
37. Auditor's increased crying (5)
38. Man with daughters entertained by two unknowns for money (5)
40. 10 and A of diamonds flipped over (4)
42. Hall's partner picked up some cereal (4)
43. King of the blues is noble (4)
44. Father's Day promotion (3)
47. Album containing single getting impertinent responses (3)
