Quick pint following concert (6)
I believe the answer is:
prompt
'quick' is the definition.
(I know that quick can be written as prompt)
'pint following concert' is the wordplay.
'pint' becomes 'pt' (abbreviation).
'following' means one lot of letters go next to another.
'concert' becomes 'prom' (abbreviation for promenade concert).
'pt' put after 'prom' is 'PROMPT'.
(Other definitions for prompt that I've seen before include "Ready" , "Precisely on time" , "Bring about; done without delay" , "Dead on time" , "Cue given to a performer" .)