Question wife on chicken (4)
I believe the answer is:
when
'question' is the definition.
(when is a question word)
'wife on chicken' is the wordplay.
'wife' becomes 'w' (abbreviation).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'chicken' becomes 'hen' (hen is a kind of chicken).
'w'+'hen'='WHEN'
(Other definitions for when that I've seen before include "Over what period?" , "Say it when you''ve had enough" , "As soon as" , "stop pouring the drink" , "At what time?" .)