Fitness fanatic consumes a little bit of a snack (6)
I believe the answer is:
peanut
'a snack' is the definition.
(I've seen this before)
'fitness fanatic consumes a' is the wordplay.
'fitness' becomes 'pe' (I can't justify this - if you can you should give a lot more credence to this answer).
'fanatic' becomes 'nut' (I've seen this before).
'consumes' is an insertion indicator.
'pe'+'nut'='penut'
'penut' enclosing 'a' is 'PEANUT'.
'little bit of' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for peanut that I've seen before include "Ante up for this type of hard fruit" , "Edible seed, may be roasted and salted" , "It ripens in a vine pod underground" , "little bit of food" , "Eat pun for the hard fruit" .)