The States have the key to employment (3)
I believe the answer is:
use
'to employment' is the definition.
(how something is used)
'the states have the key' is the wordplay.
'the states' becomes 'US' (abbreviation for the United States).
'have' is a charade indicator (letters next to each other).
'the key' becomes 'e' (a musical key).
'us'+'e'='USE'
(Other definitions for use that I've seen before include "Employ (something)" , "Operate" , "Expend in amusement" , "Consumption" , "Application" .)