Vestment almost entirely black (3)
I believe the answer is:
alb
'vestment' is the definition.
(alb is a kind of vestment)
'almost entirely black' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'entirely' becomes 'all' (I've seen this before).
'black' becomes 'b' (as in pencil classifications).
'all' with its last letter removed is 'al'.
'al'+'b'='ALB'
(Other definitions for alb that I've seen before include "Priestly garment" , "White clerical garment" , "clerical belongings" , "Priest's long white sleeved vestment" , "Without hindrance" .)