Written agreement to reduce size (8)
I believe the answer is:
contract
This clue is a double definition.
'written agreement' is the first definition.
(contract is a kind of written agreement)
'reduce size' is the second definition.
(to contract is to reduce in size)
'to' acts as a link.
(Other definitions for contract that I've seen before include "Promise" , "type of bridge" , "bargain" , "Arrangement to commit murder" , "Agree - get smaller" .)