Artist fellow is a gas! ... (5)
I believe the answer is:
radon
'a gas' is the definition.
(radon is a kind of gas)
'artist fellow' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'fellow' becomes 'don' (I've seen this before**).
'ra'+'don'='RADON'
'is' acts as a link.
(Other definitions for radon that I've seen before include "Noble gas, Rn" , "Natural gas considered a hazard to health" , "Inert gas that occurs naturally" , "Radioactive gas, linked to granite" , "Colourless gas, atomic number 86" .)