Date exclusively (2,6)
I believe the answer is:
go steady
'date exclusively' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for go steady that I've seen before include "Keep seeing each other" , "Court romantically" , "Date one person regularly" , "Have relationship as regular boy or girl friend - sounds stable" , "See someone regularly" .)