Knight on horse, battling for right (6)
I believe the answer is:
kosher
'right' is the definition.
(thesaurus)
'knight on horse battling' is the wordplay.
'knight' becomes 'k' ().
'on' means one lot of letters goes inside another.
'battling' indicates anagramming the letters (I've seen 'battle' mean this (the letters disrupted as if in battle)).
'horse' anagrammed gives 'osher'.
'k' placed into 'osher' is 'KOSHER'.
'for' is the link.
(Other definitions for kosher that I've seen before include "Correct ? legitimate" , "Ritually clean (food)" , "Pure (Jewish)" , "strict rules" , "Of food, conforming to Jewish dietary laws" .)