Propane mixture seems feasible in theory (2,5)

I believe the answer is:

on paper

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'in theory' is the definition.
(synonyms)

'propane mixture seems feasible' is the wordplay.
'propane' anagrams to 'ON PAPER'.
But, I'm not sure how this is indicated.

Can you help me to learn more?

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

I've seen this clue in The Irish Times.

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