Signed agreement? (3)
I believe the answer is:
nod
'agreement?' is the definition.
(a nod indicates agreement)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for nod that I've seen before include "Assenting head gesture" , "Agree non-verbally" , "Careless mistake" , "Show recognition" , "It's said to be as good as a wink" .)