Occasionally great fellow produces gas (5)
I believe the answer is:
radon
'gas' is the definition.
(I've seen this before)
'occasionally great fellow' is the wordplay.
'occasionally' means one should take alternating letters.
'fellow' becomes 'don' (both can mean an academic).
The alternating letters of 'great' are 'ra'.
'ra'+'don'='RADON'
'produces' acts as a link.
(Other definitions for radon that I've seen before include "Inert gas - adorn (anag)" , "Naturally occurring inert gas" , "Noble gas, Rn" , "Inert gas that occurs naturally" , "Gaseous element, Rn" .)