Silent assent (3)
I believe the answer is:
nod
'silent assent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nod that I've seen before include "Signify agreement" , "What Homer may sometime do" , "Shake head in agreement" , "Indicate 25 [YES]" , "Go to sleep in this land" .)