Defeat old-fashioned party (5)
I believe the answer is:
outdo
'defeat' is the definition.
(to outdo someone is to better or defeat them)
'old-fashioned party' is the wordplay.
'old-fashioned' becomes 'out' (as opposed to 'in').
'party' becomes 'do' (do is a kind of party).
'out'+'do'='OUTDO'
(Other definitions for outdo that I've seen before include "Eclipse" , "going to top?" , "Overcome" , "Get the better of" , "Perform better than surpass" .)