Prohibition almost ended: exactly (4,2)
I believe the answer is:
bang on
'exactly' is the definition.
(as a response to a true statement)
'prohibition almost ended' is the wordplay.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'almost' means to remove the last letter (most of the word but not all of it).
'ended' becomes 'gone' (going is a kind of ending).
'gone' with its final letter removed is 'gon'.
'ban'+'gon'='BANG ON'
(Other definitions for bang on that I've seen before include "Talk at length" , "Exactly right - sounds noisy" , "Keep talking tediously" , "Absolutely right, just so - in noisy way?" , "Informally, absolutely correct - may be noisy" .)