I interrupt to oust petitioner (9)
I believe the answer is:
suppliant
'petitioner' is the definition.
('suppliant' can be a synonym of 'petitioner')
'i interrupt to oust' is the wordplay.
'interrupt' indicates putting letters inside.
'to oust' becomes 'supplant' (I've seen this before).
'i' placed within 'supplant' is 'SUPPLIANT'.
(Other definitions for suppliant that I've seen before include "One who begs humbly" , "One begging" , "Asking earnestly" , "One praying for a favour" , "One making an appeal" .)