This is a cryptic crossword puzzle. Clues are normal, however some entries clash with one another. Each such clash is resolved by adding the clashing letters using the standard A1-Z26 numerology, subtracting 26 as needed and converting back to a letter. If a clash should fall on a shaded square then it should be entered in lower case; otherwise it should be entered in upper-case. Once the grid is complete the solver should clear their board, interpret the clashes appropriately for the puzzle title, and complete the puzzle. The key move required for puzzle completion should be given in order to obtain the tick.
Across: 1. One-handed God hangs heads of Thor and Ratatosk about crown of Yggdrasil (3) 3. A standard for comparison is cast down (5) 8. I left the Aegis, confused by idiosyncratic Celtic taboo (4) 11. Managed unlimited grant (3) 12. Postpone work on ruined lamb-holt (8) 13. Turn inside out to prevent article from becoming 'European' (5) 15. Quetelet's studies provided second-rate military intelligence (3) 16. Fight for the carriage driver's seat (3) 17. Man, after helpful association with Jewish High Priest (5) 18. Lift cover on clearly understood subject (4,4) 21. Small marker buoy -- and at sea (3) 22. Instructions brother exchanged with student (willingly) (4) 23. Shady enclosure in gazebo we repainted (5) 24. Father (Doctor of Divinity) grasping answer (3) Down: 1. Delicate situation is waiting for the right moment to explode (4,4) 2. Esteemed hunter ate dragon heart (5) 4. Abbot Li vaguely cut short... (7) 5. ...human forelimb; cut short injury (3) 6. Detail a fish and its taxonomy (5) 7. Closely connected: Lenin and Derek mingled without hesitation (8) 9. 20 returns running water (2) 10. Stifle non-Liberal, not one's own (7) 14. A year after peak, yield (2,3) 15. Embroidery: a bird in flight (5) 19. Compass heading shown by the needle (3) 20. Another missing North -- Ian's one (2)
(All answers can be found in Chambers Dictionary except for 8 across, which is easily found on the web)