US city custom, a habit to some extent (5)
I believe the answer is:
omaha
'us city' is the definition.
(I've seen this before)
'custom a habit to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'OMAHA' can be found hidden inside 'custom a habit'.
(Other definitions for omaha that I've seen before include "place in Nebraska" , "US city; D-Day 16" , "Largest city in Nebraska" , "American-Indians" , "Cotton fabric used for curtains and upholstery" .)