Knight finally takes one rook (3)
I believe the answer is:
sir
'knight' is the definition.
(a knight has the title Sir)
'finally takes one rook' is the wordplay.
'finally' indicates one should take the final letters.
'one' becomes 'i' (Roman numeral).
'rook' becomes 'R' (chess abbreviation).
The last letter of 'takes' is 's'.
's'+'i'+'r'='SIR'
(Other definitions for sir that I've seen before include "Honorary male title" , "Title for knight or teacher" , "Title of 1 across" , "Formal term of address" , "Gentleman's form of address" .)