Reformed criminal leader sent to the gallows (7)
I believe the answer is:
changed
'reformed' is the definition.
(reform means to change or improve)
'criminal leader sent to the gallows' is the wordplay.
'leader' suggests taking the first letters.
'sent to the gallows' becomes 'hanged' (I've seen this in another clue).
The initial letter of 'criminal' is 'c'.
'c'+'hanged'='CHANGED'
(Other definitions for changed that I've seen before include "Modified" , "Turned" , "Different" , "Altered - substituted" .)