In theory, propane explodes (2,5)
I believe the answer is:
on paper
'in theory' is the definition.
('on paper' can be a synonym of 'in theory')
'propane explodes' is the wordplay.
'explodes' indicates an anagram.
'propane' with letters rearranged gives 'ON PAPER'.
(Other definitions for on paper that I've seen before include "As written - in theory" , "Hypothetically" , "Exam may be" , "Theoretical" , "Planned (in theory)" .)