Live in sin, habitually? To an extent (7)
I believe the answer is:
inhabit
'live in' is the definition.
(to inhabit a place is to live in it)
'sin habitually? to an extent' is the wordplay.
'to an extent' says the answer is hidden in the clue.
'INHABIT' can be found hidden inside 'sin habitually'.
(Other definitions for inhabit that I've seen before include "Occupy - settle" , "Live in (a place)" , "people" , "Tenant" , "Dwell in" .)