Party girl initially aggressive (4)
I believe the answer is:
gala
'party' is the definition.
(I know that party can be written as gala)
'girl initially aggressive' is the wordplay.
'girl' becomes 'gal' (synonyms).
'initially' says to take the initial letters.
The initial letter of 'aggressive' is 'a'.
'gal'+'a'='GALA'
(Other definitions for gala that I've seen before include "Festive entertainment" , "Public festivity" , "Special festivity" , "Jamboree" , "Festive event" .)