Cook too much, party having finished earlier (6)
I believe the answer is:
overdo
'cook too much' is the definition.
(I've seen this before)
'having finished earlier' is the wordplay.
'having' becomes 'do' (I can't justify this - if you can you should give a lot more credence to this answer).
'finished' becomes 'over' ('over' can be similar in meaning to 'finished').
'earlier' says to put letters next to each other.
'do' after 'over' is 'OVERDO'.
'party' acts as a link.
This may not be right. It may belong to another bit of the clue.
(Other definitions for overdo that I've seen before include "Carry too far" , "Cook too long or act excessively" , "Work too much" , "Be excessive in action or cooking" , "Exaggerate; use too much of" .)