A large, black religious garment (3)
I believe the answer is:
alb
'religious garment' is the definition.
'alb' can be an answer for 'garment' (I have seen 'Clerical garment ' mean 'alb' so perhaps 'garment' could also mean 'alb'). I am not sure about the 'religious' bit.
'a large black' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'black' becomes 'b' (as in pencil classifications).
'a'+'l'+'b'='ALB'
Can you help me to learn more?
(Other definitions for alb that I've seen before include "the vicar might put on?" , "Long white church vestment" , "Clerical gown" , "White linen liturgical vestment" , "White garment worn by priests" .)