Priest's vestment (7)
(Other definitions for cassock that I've seen before include "Long clerical garment" , "Is it a habit of the clergy" , "Clerical garb" , "Chorister's robe" , "Long clerical robe" .)
'priest's vestment' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alb that I've seen before include "Long white church vestment" , "White clerical garment" , "Cover" , "clerical belongings" , "Clerical gown" .)