Reason I'll get stuck into health food (5)
I believe the answer is:
brain
'reason' is the definition.
(thesaurus)
'stuck into health food' is the wordplay.
'stuck' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'into' is an insertion indicator.
'health food' becomes 'bran' (I've seen this before).
'i' inserted into 'bran' is 'BRAIN'.
'i'll get' is the 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 brain that I've seen before include "Intelligent person" , "Thinking thing" , "Intellectual ability" , "Organ of the intellect" , "Clever type" .)