Custom at home (7)
I believe the answer is:
habitat
'home' is the definition.
(thesaurus)
'custom at' is the wordplay.
'custom' becomes 'habit' (habit is a kind of custom).
'habit'+'at'='HABITAT'
(Other definitions for habitat that I've seen before include "home, naturally" , "Natural home of a plant or animal" , "Natural home of an organism" , "One's normal environment" , "Where something's usually found" .)