Witnesses a key agreement (4)
I believe the answer is:
eyes
'witnesses' is the definition.
(I've seen this before)
'a key agreement' is the wordplay.
'a key' becomes 'e' (I've seen this in other clues**).
'agreement' becomes 'yes'.
'e'+'yes'='EYES'
(Other definitions for eyes that I've seen before include "Regards -- features of a potato" , "The first things children open on Christmas Day" , "Globular organs" , "Narrowly watches" , "private detectives" .)