Girl having a field day (4)
I believe the answer is:
gala
'field day' is the definition.
(I've seen this before)
'girl having a' is the wordplay.
'girl' becomes 'gal' ('gal' can be a synonym of 'girl'**).
'having' means one lot of letters go next to another.
'gal'+'a'='GALA'
(Other definitions for gala that I've seen before include "sporting event" , "Special social or sporting occasion" , "Festivity - celebration" , "Special festivity" , "Jamboree" .)