Gym fanatic consuming a source of protein (6)
I believe the answer is:
peanut
'source of protein' is the definition.
'gym fanatic consuming a' is the wordplay.
'gym' becomes 'pe'.
'fanatic' becomes 'nut' (I've seen this before).
'consuming' is an insertion indicator.
'pe'+'nut'='penut'
'penut' going around 'a' is 'PEANUT'.
(Other definitions for peanut that I've seen before include "oil-producing plant" , "Popular edible seed, often salted" , "Edible seed, may be roasted and salted" , "little bit of food" , "It ripens in a vine pod underground" .)