Process food, good in various diets (6)
I believe the answer is:
digest
'process food' is the definition.
(I've seen this before)
'good in various diets' is the wordplay.
'good' becomes 'g' (abbreviation).
'in' is an insertion indicator.
'various' is an anagram indicator.
'diets' with letters rearranged gives 'diest'.
'g' placed into 'diest' is 'DIGEST'.
(Other definitions for digest that I've seen before include "Periodical summarizing the news" , "Make more concise" , "Long walk, itinerant person" , "book" , "Absorb, food or information" .)