It's a question of timing (4)
I believe the answer is:
when
'it's a question of timing' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for when that I've seen before include "Say it when you''ve had enough" , "As soon as" , "At what time?" , "Over what period?" , "stop pouring the drink" .)