A party activity (3)
I believe the answer is:
ado
'activity' is the definition.
(ado can mean commotion or activity)
'a party' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'a'+'do'='ADO'
(Other definitions for ado that I've seen before include "Rapid bustling commotion" , "Hoo-ha" , "Fuss, trouble" , "Fuss, bother" , "Flap" .)