Agree but say nothing (3)
I believe the answer is:
nod
'agree' is the definition.
(I've seen this before)
'say nothing' is the wordplay.
I cannot really understand how this works, but
'nothing' could be 'no' (similar in meaning) and 'no' is found in the answer.
A single letter 'd' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'but' acts as a link.
Can you help me to learn more?
(Other definitions for nod that I've seen before include "Acknowledge by gesture" , "sleepy land" , "overlook something" , "Cain was banished to the land of ..." , "Drop the head" .)