Cockney cads — slippery types (4)
I believe the answer is:
eels
I believe this clue is a double definition.
'cockney cads' is the first definition.
('heels' - 'heel' can be a synonym of 'cad' - said with a cockney accent, dropping the 'H')
'slippery types' is the second definition.
(slippery as an eel)
(Other definitions for eels that I've seen before include "Slippery customers" , "Abstract nineteen sixties style" , "Congers" , "Anguillae" , "Sea snakes among lees" .)