Couple beginning to want closed borders (3)
I believe the answer is:
two
'couple' is the definition.
(two people or things)
'beginning to want closed borders' is the wordplay.
'beginning to' indicates taking the first letters.
'closed' becomes 'to' (as in a door closed to).
'borders' means one lot of letters goes inside another (inserted letters are bordered by others).
The initial letter of 'want' is 'w'.
'w' inserted within 'to' is 'TWO'.
(Other definitions for two that I've seen before include "Cat's coat" , "Old song 'Tea for . . .'" , "Number of Gentlemen of Verona" , "prime -- that's not odd" , "Number of faces of the god Janus" .)