Going off grit, not used (7)
I believe the answer is:
rotting
'going off' is the definition.
The answer and definition can be both related to change as well as being present participle verbs.
Perhaps there's an association between them I don't understand?
'grit not used' is the wordplay.
'used' indicates an anagram.
'grit'+'not'='gritnot'
'gritnot' anagrammed gives 'ROTTING'.
Can you help me to learn more?
(Other definitions for rotting that I've seen before include "Sultanate in NW Borneo" , "Decaying" , "corruption?" , "Putrid" , "Going bad" .)