Certainly not the best policy! (10)
I believe the answer is:
dishonesty
'certainly not the best policy' is the definition.
I can't tell whether this defines the answer.
'not the best policy' is the wordplay.
'not' becomes 'dis' (I am not sure about this - if you are sure you should believe this answer much more).
'the best policy' becomes 'honesty' (I've seen this in another clue).
'dis'+'honesty'='DISHONESTY'
Can you help me to learn more?
(Other definitions for dishonesty that I've seen before include "fraud" , "Crookedness" , "Bad behaviour" , "Graft" , "Untrustworthiness, insincerity" .)