No-goodnik (3)
'no-goodnik' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rat that I've seen before include "Inform" , "Long-tailed rodent" , "one escaping doomed vessel?" , "Treacherous rodent" , "one who deserts the sinking ship?" .)
(Other definitions for cad that I've seen before include "An Englishman who behaves dishonourably, a rotter" , "Blackguard" , "One behaves dishonourably" , "Wretch" , "A knave, real rotter" .)