Stir it into stone (7)
I believe the answer is:
agitate
'stir' is the definition.
('agitate' can be a synonym of 'stir')
'it into stone' is the wordplay.
'into' means one lot of letters goes inside another.
'stone' becomes 'agate' (I've seen this before).
'it' put inside 'agate' is 'AGITATE'.
(Other definitions for agitate that I've seen before include "Stir up (trouble?)" , "Unsettle" , "Campaign" , "Stir vigorously" , "Stir violently" .)