Best blooming party ... (5)
I believe the answer is:
outdo
'best' is the definition.
(I know that best can be written as outdo)
'blooming party' is the wordplay.
'blooming' becomes 'out' (plants which have bloomed are 'out').
'party' becomes 'do' (do is a kind of party).
'out'+'do'='OUTDO'
(Other definitions for outdo that I've seen before include "Perform better than, be superior to" , "going to top?" , "Overtake" , "Br superior to (someone)" , "Best" .)