Propane explodes, theoretically (2,5)
I believe the answer is:
on paper
'theoretically' is the definition.
(synonyms)
'propane explodes' is the wordplay.
'explodes' is an anagram indicator.
'propane' with letters rearranged gives 'ON PAPER'.
(Other definitions for on paper that I've seen before include "As written - in theory" , "Exam may be" , "In writing" , "Where one might write" , "Theoretical" .)