Lowest of the low
'lowest of the low' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for worst that I've seen before include "Direst" , "Beat" , "Most badly" , "Strangely, best" , "the pits" .)
(Other definitions for bass that I've seen before include "Fish - low voice" , "Fish for the low-voiced" , "Deep-voiced male singer" , "A fish, or the lowest pitch in music" , "Type of fish" .)