Lowest of the low? (4)
'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" .)