Criminal's sort of rail about reformed peers (9)
I believe the answer is:
desperado
'criminal's' is the definition.
(desperado is a kind of criminal)
'sort of rail about reformed peers' is the wordplay.
'sort of rail' becomes 'dado' (as in a dado rail, an architectural feature).
'about' is an insertion indicator.
'reformed' indicates anagramming the letters.
'peers' is an anagram of 'esper'.
'dado' placed around 'esper' is 'DESPERADO'.
(Other definitions for desperado that I've seen before include "reckless type" , "Road speed (anag.)" , "Reckless person" , "Reckless guy" , "Bandit, outlaw" .)