Pope ran off, in theory (2,5)
I believe the answer is:
on paper
'in theory' is the definition.
(synonyms)
'pope ran off' is the wordplay.
'off' indicates anagramming the letters.
'pope'+'ran'='poperan'
'poperan' with letters rearranged gives 'ON PAPER'.
(Other definitions for on paper that I've seen before include "In writing" , "Exam may be" , "Planned (in theory)" , "Theoretical" , "Propane (anag.)" .)