New form of propane, in theory (2,5)
I believe the answer is:
on paper
'in theory' is the definition.
('on paper' can be a synonym of 'in theory')
'new form of propane' is the wordplay.
'new form of' is an anagram indicator.
'propane' with letters rearranged gives 'ON PAPER'.
(Other definitions for on paper that I've seen before include "Where Times staff work" , "Exam may be" , "Where one might write" , "Propane (anag.)" , "Theoretically" .)