Black art, possibly? (3)
I believe the answer is:
tar
'black' is the definition.
(I've seen this in other clues)
'art possibly?' is the wordplay.
'possibly?' is an anagram indicator (I've seen 'possibly' mean this).
'art' anagrammed gives 'TAR'.
(Other definitions for tar that I've seen before include "Liquid from wood, coal" , "Viscous bituminous substance" , "Sailor; bitumen" , "one may need to pay on deck" , "Product of coal" .)