Stop pouring whiskey on bird (4)
I believe the answer is:
when
'stop pouring' is the definition.
(said to ask someone to stop pouring a drink)
'whiskey on bird' is the wordplay.
'whiskey' becomes 'W' (alpha bravo charlie delta ...).
'on' means one lot of letters go next to another.
'bird' becomes 'hen' (hen is a kind of bird).
'w'+'hen'='WHEN'
(Other definitions for when that I've seen before include "At what time?" , "Say it when you''ve had enough" , "As soon as" , "stop pouring the drink" , "Over what period?" .)