Cap or old hat found before party (5)
I believe the answer is:
outdo
'cap' is the definition.
(I've seen this before)
'old hat found before party' is the wordplay.
'old hat' becomes 'out' (both can mean old-fashioned or outdated).
'found before' is a charade indicator (letters next to each other).
'party' becomes 'do' (do can informally mean a party).
'out'+'do'='OUTDO'
'or' is the link.
(Other definitions for outdo that I've seen before include "Perform better than another" , "Br superior to (someone)" , "Surpass - overshadow - excel" , "going to top?" , "Defeat" .)