Most definitely not good for one (3,3)
I believe the answer is:
ill say
'most definitely' is the definition.
('I'll say' can show enthusiastic agreement)
'not good for one' is the wordplay.
'not good' becomes 'ill' ('ill' is the opposite to 'good').
'for one' becomes 'say' (both can mean 'for example').
'ill'+'say'='ILL SAY'
(Other definitions for ill say that I've seen before include "You bet" , "Injuries" , "I couldn't agree more!" , "Rather!" , "I totally agree!" .)