Pair set to reform, criminals in the main (7)
I believe the answer is:
pirates
'criminals in the main' is the definition.
(the main is an old name for the sea)
'pair set to reform' is the wordplay.
'to reform' indicates anagramming the letters (letters in a new form).
'pair'+'set'='pairset'
'pairset' anagrammed gives 'PIRATES'.
(Other definitions for pirates that I've seen before include "Bad sailors" , "Robbers on the high seas" , "More than one brigand" , "Copyright infringers" , "felons found in craft?" .)