Second rook capturing one knight (3)
I believe the answer is:
sir
'knight' is the definition.
(a knight has the title Sir)
'second rook capturing one' is the wordplay.
'second' becomes 's' (common abbreviation).
'rook' becomes 'R' (chess abbreviation).
'capturing' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
's'+'r'='sr'
'sr' enclosing 'i' is 'SIR'.
(Other definitions for sir that I've seen before include "Title given to 10 Across" , "Male title of respect" , "Poet and novelist" , "Title given to baronet" , "Polite or formal address to male" .)