Assents silently (4)
I believe the answer is:
nods
'assents silently' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nods that I've seen before include "Non-verbal indications of assent" , "gestures in agreement" , "Inclines head" , "is about to crash?" , "Makes mistake" .)