Low singer in club assignment (4)
I believe the answer is:
'low singer' is the definition.
(I've seen this before)
'in club assignment' is the wordplay.
'in' says the answer is hidden in the clue.
'BASS' can be found hidden inside 'club assignment'.
(Other definitions for bass that I've seen before include "Booming percussion instrument" , "Fish - low in pitch" , "Lowest male singer" , "Performer" , "Fish - low voice" .)