Regards as a key agreement (4)
I believe the answer is:
eyes
'regards' 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'
'as' acts as a link.
(Other definitions for eyes that I've seen before include "Organs of vision" , "Looks at carefully" , "The first things children open on Christmas Day" , "Spots on a peacock's tail" , "Ogles" .)