Long priestly vestment, a pound (3)
I believe the answer is:
alb
'long priestly vestment' is the definition.
'alb' can be an answer for 'vestment' (alb is a kind of vestment). I'm not certain of the 'long priestly' bit.
'a pound' is the wordplay.
'pound' becomes 'lb' (synonyms).
'a'+'lb'='ALB'
Can you help me to learn more?
(Other definitions for alb that I've seen before include "Priestly garment" , "White linen liturgical vestment" , "the vicar might put on?" , "ecclesiastical garment" , "Clerical gown" .)