Quick pint after concert (6)
I believe the answer is:
prompt
'quick' is the definition.
('prompt' can be a synonym of 'quick')
'pint after concert' is the wordplay.
'pint' becomes 'pt'.
'after' is a charade indicator (letters next to each other).
'concert' becomes 'prom' (abbreviation for promenade concert).
'pt' put after 'prom' is 'PROMPT'.
(Other definitions for prompt that I've seen before include "Incite - punctual" , "Bring about; actors' help" , "Cause; on time" , "Punctual or timely" , "On the dot" .)