Replace petitioner after one leaves (8)
I believe the answer is:
supplant
'replace' is the definition.
(synonyms)
'petitioner after one leaves' is the wordplay.
I cannot really understand how this works, but
'one' could be 'an' ('a' and 'an' can mean 'one') and 'an' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for supplant that I've seen before include "Displace" , "take another's place" , "Supersede and replace" , "Oust" .)