Knight is backing king (3)
I believe the answer is:
sir
'knight' is the definition.
(I've seen this before)
'is backing king' is the wordplay.
'backing' says the letters should be written backwards.
'king' becomes 'r' (r is abbreviation for rex).
'is' reversed gives 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Respectful address for a man" , "... ___ John Major" , "Knight's prefix" , "Honorary male title" , "Term of address for a gentleman" .)