Ham finished before a case of claret (7)
I believe the answer is:
overact
'ham' is the definition.
('overact' can be a synonym of 'ham')
'finished before a case of claret' is the wordplay.
'finished' becomes 'over' ('over' can be similar in meaning to 'finished').
'before' says to put letters next to each other.
'case of' suggests removing the centre (outsides of).
'claret' with its middle taken out is 'ct'.
'over'+'a'+'ct'='OVERACT'
(Other definitions for overact that I've seen before include "Be excessive on stage" , "Do what ham thespian does" , "Play (part) in an exaggerated manner" , "pile it on" , "In part, fail to convince" .)