Silent affirmation (3)
I believe the answer is:
nod
'silent affirmation' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nod that I've seen before include "Indicate assent; make slip" , "Agree non-verbally" , "Green light" , "Bob the head" , "Appear to fall asleep" .)