Black bits found in butcher's mutton (4)
I believe the answer is:
smut
'black bits' is the definition.
'in butcher's mutton' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SMUT' can be found hidden inside 'butchers mutton'.
'found' is the link.
(Other definitions for smut that I've seen before include "blue material" , "Piece of dirt or salacious talk" , "Small soot flake or indecent talk" , "Soot, dirt, grit" , "porn?" .)