Don't apply to O2 for one (5)
I believe the answer is:
arena
'don't apply' is the definition.
I can't judge whether this defines the answer.
'o2 for one' is the wordplay.
'oe' becomes 'rena' (I am not sure about this - if you are sure you should believe this answer much more).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'one' becomes 'a' (a thing is one thing).
'rena' after 'a' is 'ARENA'.
'to' is the link.
Can you help me to learn more?
(Other definitions for arena that I've seen before include "Place of great activity" , "An era (anag.)" , "Venue for boxing, circus and other action events" , "Sphere of action" , "Wembley, for example" .)