Propane volatile in theory (2,5)

I believe the answer is:

on paper

'in theory' is the definition.
(synonyms)

'propane volatile' is the wordplay.
'volatile' is an anagram indicator.
'propane' anagrammed gives 'ON PAPER'.

(Other definitions for on paper that I've seen before include "Where one might write" , "As written - in theory" , "Theoretically" , "Propane (anag.)" , "In writing" .)

I've seen this clue in The Telegraph.

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