Gas flowed to maintain function (5)
I believe the answer is:
radon
'gas' is the definition.
(thesaurus)
'flowed to maintain function' is the wordplay.
'flowed' becomes 'ran' ('run' can be a synonym of 'flow').
'to maintain' indicates putting letters inside.
'function' becomes 'do' (I've seen this in other clues).
'ran' placed around 'do' is 'RADON'.
(Other definitions for radon that I've seen before include "Gaseous element, Rn" , "Inert gas that occurs naturally" , "Rare noble gas" , "gets no reaction" , "Colourless gas, atomic number 86" .)