Approve party in apartment (5)
I believe the answer is:
adopt
'approve' is the definition.
(thesaurus)
'party in apartment' is the wordplay.
'party' becomes 'do' (do can informally mean a party).
'in' is an insertion indicator.
'apartment' becomes 'apt' (abbreviation in property ads for example).
'do' going within 'apt' is 'ADOPT'.
(Other definitions for adopt that I've seen before include "Formally approve" , "Foster" , "Take on (another's child)" , "Take up, follow" , "Acquire" .)