Prohibition? Gee, a blow! (4)
I believe the answer is:
bang
'a blow' is the definition.
(bang is a kind of blow)
'prohibition? gee' is the wordplay.
'prohibition?' becomes 'ban' (ban is a kind of prohibition).
'gee' becomes 'g' ('g' can be a synonym of 'gee').
'ban'+'g'='BANG'
(Other definitions for bang that I've seen before include "Sudden sharp noise" , "Sudden impact" , "Explosive noise" , "hit" , "Gun sound" .)