Clergyman's gown (7)
I believe the answer is:
cassock
'clergyman's gown' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cassock that I've seen before include "Long garment worn by some clergy" , "Long clerical garment" , "Cossack (anag.)" , "Clerical garb" , "Clergyman's robe" .)