Replace petitioner? Not I (8)
I believe the answer is:
supplant
'replace' is the definition.
('supplant' can be a synonym of 'replace')
'petitioner? not i' is the wordplay.
'petitioner?' becomes 'suppliant' ('suppliant' can be a synonym of 'petitioner').
'not' indicates named letters should be taken away.
'suppliant' with 'i' removed is 'SUPPLANT'.
(Other definitions for supplant that I've seen before include "take another's place" , "Displace" , "Oust" , "Supersede and replace" .)