Employs you and I to test core (4)
I believe the answer is:
uses
'employs' is the definition.
('use' can be a synonym of 'employ')
'you and i to test core' is the wordplay.
'you and i' becomes 'us' (the two of us).
'core' indicates the central letters.
The central letters of 'test' are 'es'.
'us'+'es'='USES'
(Other definitions for uses that I've seen before include "deployments" , "What an employer does" , "Customs" , "Operates" , "Exercises" .)