Live in costume (7)
I believe the answer is:
inhabit
'live' is the definition.
(I know that live can be written as inhabit)
'in costume' is the wordplay.
'costume' becomes 'habit' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in'+'habit'='INHABIT'
Can you help me to learn more?
(Other definitions for inhabit that I've seen before include "Live in, occupy" , "Occupy (dwelling)" , "Tenant" , "Dwell in" , "Live in (a place)" .)