Low singer in dub assignment (4)
I believe the answer is:
bass
'low singer' is the definition.
(I've seen this before)
'in dub assignment' is the wordplay.
'in' says the answer is hidden in the clue.
'BASS' can be found hidden inside 'dub assignment'.
(Other definitions for bass that I've seen before include "Scottish rock" , "Fish, guitar" , "Low singing voice" , "part of score" , "Fish; vocalist" .)