Grit sac evolved — it's involved in digestion (7)
I believe the answer is:
gastric
'involved in digestion' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives sometimes mean the same thing.
'grit sac evolved' is the wordplay.
'evolved' indicates an anagram (the order of letters changes or evolves).
'grit'+'sac'='gritsac'
'gritsac' with letters rearranged gives 'GASTRIC'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for gastric that I've seen before include "of the corporation?" , "Pertaining to the stomach" , "Intestinal" , "affecting stomach" , "Relating to or involving the stomach" .)