Not a single tradesman is a racketeer (6-6)
I believe the answer is:
double-dealer
'a racketeer' is the definition.
(thesaurus)
'not a single tradesman' is the wordplay.
'not a single' becomes 'double' (I can't justify this - if you can you should believe this answer much more).
'tradesman' becomes 'dealer' (I've seen this in another clue).
'double'+'dealer'='DOUBLE-DEALER'
'is' is the link.
Can you help me to learn more?
(Other definitions for double-dealer that I've seen before include "Person who says one thing and does another" , "Deceitful person" , "Deceitful individual" , "Deceiver, cheat" .)