Wacky bunch in small party (9)
I believe the answer is:
screwball
'wacky' is the definition.
(I've seen this before)
'bunch in small party' is the wordplay.
'bunch' becomes 'crew' (synonyms).
'in' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'party' becomes 'ball' (ball is a kind of party).
's'+'ball'='sball'
'crew' going within 'sball' is 'SCREWBALL'.
(Other definitions for screwball that I've seen before include "touched person" , "Crazy (person)" , "Freak" , "odd fellow?" , "Eccentric" .)