Upstanding chap claiming jail reformed prophet (6)
I believe the answer is:
elijah
'prophet' is the definition.
(I've seen this before)
'upstanding chap claiming jail reformed' is the wordplay.
'upstanding' says the letters should be written in reverse.
'chap' becomes 'he'.
'claiming' is an insertion indicator.
'reformed' indicates anagramming the letters.
'he' written backwards gives 'eh'.
'jail' with letters rearranged gives 'lija'.
'eh' placed around 'lija' is 'ELIJAH'.
(Other definitions for elijah that I've seen before include "Hebrew prophet who disappeared in fiery chariot" , "OT, Islamic prophet" , "Hebrew prophet in the time of Jezebel" , "Hebrew prophet who rebuked Ahab and Jezebel" , "Biblical prophet" .)