Come down with an agreement (8)
I believe the answer is:
contract
This clue is a double definition.
'come down with' is the first definition.
('contract' can be a synonym of 'come down with')
'an agreement' is the second definition.
(contract is a kind of agreement)
(Other definitions for contract that I've seen before include "Come down with" , "Pick up" , "Shrink in legal agreement?" , "Undertake" , "four spades, perhaps" .)