Found in the slop, a herring-like fish (4)
I believe the answer is:
opah
'fish' is the definition.
(I know that Opah is a type of malacopterygian)
'found in the slop a herring' is the wordplay.
'found in' says the answer is hidden in the clue.
'OPAH' can be found hidden inside 'slop a herring'.
'like' acts as a link.
(Other definitions for opah that I've seen before include "Large deep-sea fish" , "Kingfish" , "Ocean swimmer" .)