Takes advantage of United States and Spain (4)
I believe the answer is:
'takes advantage' is the definition.
(I've seen this in other clues)
'united states and spain' is the wordplay.
'united states' becomes 'US' (abbreviation for the United States).
'and' is a charade indicator (letters next to each other).
'spain' becomes 'es' (es is an abbreviation for Spain).
'of' is the link.
(Other definitions for uses that I've seen before include "Takes (drugs)" , "Utilises" , "Employs" , "Puts into service" , "What an employer does" .)