Rustic forbidden to appear in a French church (7)
I believe the answer is:
uncouth
'rustic' is the definition.
'forbidden to appear in a french church' is the wordplay.
'forbidden' becomes 'out' ('out' can be a synonym of 'forbidden').
'to appear in' indicates putting letters inside.
'a french' becomes 'un' ('a' in French).
'church' becomes 'ch' (common abbreviation).
'un'+'ch'='unch'
'out' put inside 'unch' is 'UNCOUTH'.
(Other definitions for uncouth that I've seen before include "Gross" , "Uncultured" , "Ill-bred, unrefined" , "Ill-mannered" , "Boorish, clumsy in manner" .)