In theory, propane exploded (2,5)
I believe the answer is:
on paper
'in theory' is the definition.
(synonyms)
'propane exploded' is the wordplay.
'exploded' indicates an anagram.
'propane' with letters rearranged gives 'ON PAPER'.
(Other definitions for on paper that I've seen before include "Theoretical" , "Where one might write" , "Hypothetically" , "Exam may be" , "Planned (in theory)" .)