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' shows that the letters should be reversed in order.
'chap' becomes 'he'.
'claiming' means one lot of letters goes inside another.
'reformed' indicates anagramming the letters.
'he' back-to-front is '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" .)