Man worked with don, I bet? Definitely (3,2,7)
I believe the answer is:
and no mistake
'definitely' is the definition.
(thesaurus)
'man worked with don i bet?' is the wordplay.
'worked with' is an anagram indicator.
'bet?' becomes 'stake' ('stake' can be a synonym of 'bet').
'man' anagrammed with 'don' makes 'andnom'.
'andnom'+'i'+'stake'='AND NO MISTAKE'
(Other definitions for and no mistake that I've seen before include "Without any doubt" , "Assuredly (informal)" .)