Fruitcake interchangeable with any form of sustenance (8)
I believe the answer is:
nutrient
'sustenance' is the definition.
(sustenance can mean nutrition)
'fruitcake interchangeable with any form' is the wordplay.
I cannot really see how this works, but
'fruitcake' could be 'nut' (I've seen this before) and 'nut' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for nutrient that I've seen before include "Item of food" , "Nourishing substance" , "A food that keeps one alive and healthy" , "Substance providing nourishment" , "Source of nourishment" .)