Cocky mob suit up to party (9)
I believe the answer is:
bumptious
'cocky' is the definition.
(thesaurus)
'mob suit up to party' is the wordplay.
'to party' indicates anagramming the letters (I've seen 'parties' mean this (the letters move or party around)).
'mob'+'suit'+'up'='mobsuitup'
'mobsuitup' with letters rearranged gives 'BUMPTIOUS'.
(Other definitions for bumptious that I've seen before include "pushy?" , "Offensively self-assertive" , "Irritatingly self-assertive" , "self-important" .)