Dismissed by the party, is overcome (5)
I believe the answer is:
outdo
'is overcome' is the definition.
(I've seen this before)
'dismissed by the party' is the wordplay.
'dismissed' becomes 'out' (I've seen this before).
'by' means one lot of letters go next to another.
'the party' becomes 'do' (do is a kind of party).
'out'+'do'='OUTDO'
(Other definitions for outdo that I've seen before include "Overcome" , "going to top?" , "Perform better than, be superior to" , "Eclipse" , "Exceed" .)