One in a black suit (5)
I believe the answer is:
spade
'one in a black suit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spade that I've seen before include "Digging implement" , "Jack or one of a dozen others" , "that has a cutting edge" , "Garden tool and card suit" , "One in a suit" .)