One's elevated by king — knight's title? (3)
I believe the answer is:
sir
'knight's title?' is the definition.
(I've seen this before)
'one's elevated by king' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'elevated' says the letters should be written backwards (in a down clue, letters go up).
'by' is a charade indicator (letters next to each other).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'is' back-to-front is 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Polite or formal address to male" , "as Winston Churchill was" , "Title given to 10 Across" , "Title of knight or baronet" , "Formal and deferential address to a man" .)