Small fiddle for cad (7)
I believe the answer is:
stinker
'for cad' is the definition.
(thesaurus)
'small fiddle' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fiddle' becomes 'tinker' (I've seen this in another clue).
's'+'tinker'='STINKER'
(Other definitions for stinker that I've seen before include "Unpleasant person - difficult task" , "Difficult problem" , "Terrible job" , "no easy thing" , "Very hard task" .)