A question of time (4)
I believe the answer is:
when
This clue is a double definition.
'a question' is the first definition.
(when is a question word)
'time' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for when that I've seen before include "At what time?" , "As soon as" , "Say it when you''ve had enough" , "stop pouring the drink" , "Over what period?" .)