Girl’s keeping new belt (4)
I believe the answer is:
zone
'belt' is the definition.
(thesaurus)
'girl's keeping new' is the wordplay.
'girl' becomes 'zoe' (**).
'keeping' means one lot of letters goes inside another.
'new' becomes 'n'.
'zoe' placed around 'n' is 'ZONE'.
(Other definitions for zone that I've seen before include "Area with particular characteristics" , "One of five belts round the earth" , "Designated or defined area" , "Bitter, pungent" , "Area; belt" .)