Here are some clearer explanations for some of the clues:
CAUTION: This post may contain spoilers or Wikipedia links.
(not sure yet)
Oilstone regularly contains finest variant of element (7)
Essential metal extracted from almost offensive hormone (7)
No back route, proceeding as planned (2 5)
A car holds freight items securely in place (7)
(not sure ...
13. While offense may be overlooked, repeat offenders will be banned (7)
14. The guard started to ring the alarm (7)
15. Stuffing into a stereo appliance (7)
19. A surprisingly mature beginner (7)
20. A spit fed on the runway (7)
21. Street next to singer Blunt's orange property (2 5)
25. Which of thesis well ...