It's the custom at home (7)
I believe the answer is:
habitat
'home' is the definition.
(thesaurus)
'it's the custom at' is the wordplay.
'it's the custom' becomes 'habit' (habit is a kind of custom).
'habit'+'at'='HABITAT'
(Other definitions for habitat that I've seen before include "Natural environment, plant or animal" , "Surrender" , "Natural home and surroundings" , "Living environment" , "Natural home of an organism" .)