Knaves have part rewritten for lean meat eater (4,5)
I believe the answer is:
jack sprat
'lean meat eater' is the definition.
(as in the nursery rhyme)
'knaves have part rewritten' is the wordplay.
'knaves' becomes 'jacks' ('jack' can be a synonym of 'knave').
'have' is a charade indicator (letters next to each other).
'rewritten' is an anagram indicator.
'part' with letters rearranged gives 'prat'.
'jacks'+'prat'='JACK SPRAT'
'for' is the link.
(Other definitions for jack sprat that I've seen before include "He ate only lean" , "He ate no fat" , "One leaving fat" , "Nursery-rhyme character" , "he stuck to a fat-free diet" .)