Priest's cassock (7)
I believe the answer is:
soutane
'priest's cassock' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for soutane that I've seen before include "Garment for a priest" , "Catholic priest's cassock" , "item of clothing" , "Long plain robe for a priest" , "Garb for priest" .)