In theory no upsetting a daily (2,5)
I believe the answer is:
on paper
'in theory' is the definition.
('on paper' can be a synonym of 'in theory')
'no upsetting a daily' is the wordplay.
'upsetting' shows that the letters should be reversed in order.
'a daily' becomes 'paper' (newspaper).
'no' backwards is 'on'.
'on'+'paper'='ON PAPER'
(Other definitions for on paper that I've seen before include "Propane (anag.)" , "Where one might write" , "Where Times staff work" , "Planned (in theory)" , "Exam may be" .)