Jack and Les are aristocrats (6)
I believe the answer is:
nobles
'are aristocrats' is the definition.
(I've seen this in another clue)
'jack and les' is the wordplay.
'jack' becomes 'nob' (term in the card game cribbage).
'and' is a charade indicator (letters next to each other).
'nob'+'les'='NOBLES'
(Other definitions for nobles that I've seen before include "Aristocrats" , "Members of the aristocracy" , "Former Eng. gold coins" , "Men of high rank" , "Members of Lords" .)