Pub not closing before a half's drunk very quickly (2,1,5)
I believe the answer is:
in a flash
'very quickly' is the definition.
(I've seen this before)
'pub not closing before a half's drunk' is the wordplay.
'pub' becomes 'inn' (I've seen this before).
'not closing' means to remove the last letter.
'before' means one lot of letters go next to another.
'half's drunk' becomes 'flash' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'inn' with its last letter removed is 'in'.
'in'+'a'+'flash'='IN A FLASH'
(Other definitions for in a flash that I've seen before include "Very quickly" , "Almost instantly" , "How lightning appears" , "Instantly, straightaway" , "pictures may be taken" .)