Rock - it's set in stone (7)
I believe the answer is:
agitate
'rock' is the definition.
(thesaurus)
'it's set in stone' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues).
'set in' indicates putting letters inside.
'stone' becomes 'agate' (I've seen this before).
'it' put into 'agate' is 'AGITATE'.
(Other definitions for agitate that I've seen before include "Stir; tie a tag (anag.)" , "Make troubled or nervous" , "Upset, disturb the surface of" , "Stir vigorously" , "Stir; arouse" .)