Agreed! And, if we're taking turns, I'm off (5,2)
I believe the answer is:
youre on
This clue is a double definition.
'agreed' is the first definition.
('you're on' can be used to agree to a challenge)
'if we're taking turns i'm off' is the second definition.
(if I am off, then you are on)
'and' acts as a link.
(Other definitions for youre on that I've seen before include "I accept the offer you have made!" , "I agree to your terms!" , "I accept the terms" , "Go for second person" , "said to clinch a deal?" .)