Quick pint on seafront (6)
I believe the answer is:
prompt
'quick' is the definition.
('prompt' can be a synonym of 'quick')
'pint on seafront' is the wordplay.
'pint' becomes 'pt'.
'on' means one lot of letters go next to another.
'seafront' becomes 'prom' (I've seen this before).
'pt' put after 'prom' is 'PROMPT'.
(Other definitions for prompt that I've seen before include "effective in expedition" , "Instantaneous" , "Reminder" , "Incite - punctual" , "Cause, bring about" .)