No-good guy (5)
I believe the answer is:
louse
'no-good guy' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for louse that I've seen before include "Wingless bloodsucker" , "Bloodsucking parasitic insect" , "scum" , "Wingless bloodsucking insect" , "Wingless parasite" .)