It's set in stone to cause trouble (7)
I believe the answer is:
agitate
'cause trouble' is the definition.
(I've seen this before)
'it's set in stone' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues).
'set in' means one lot of letters goes inside another.
'stone' becomes 'agate' (I've seen this before).
'it' placed into 'agate' is 'AGITATE'.
'to' is the link.
(Other definitions for agitate that I've seen before include "Keep moving" , "Stir (up public opinion?)" , "Stir; worry" , "'Stir, excite (7)'" , "Stir vigorously" .)