Not one question that's easy (2,7)
I believe the answer is:
no problem
'that's easy' is the definition.
'not one question' is the wordplay.
'not' becomes 'no' (I've seen this in other clues).
'one question' becomes 'problem' (problem is a kind of question).
'no'+'problem'='NO PROBLEM'
(Other definitions for no problem that I've seen before include "Easy peasy!" , "of course" , "Don't worry about it!" , "fine" .)