V sign America’s leader deployed at borders (7)
I believe the answer is:
against
'v' is the definition.
(v means versus in sports fixtures)
'sign america's leader deployed at borders' is the wordplay.
'leader' says to take the initial letters.
'deployed' indicates anagramming the letters.
'borders' is an insertion indicator (inserted letters are bordered by others).
The first letter of 'americas' is 'a'.
'sign'+'a'='signa'
'signa' with letters rearranged gives 'gains'.
'gains' inserted within 'at' is 'AGAINST'.
(Other definitions for against that I've seen before include "Not supporting" , "on the other side" , "Opposing" , "V" , "Placed in direct contact with" .)