Silently agree (3)
I believe the answer is:
nod
'silently agree' 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 "Show agreement (or sleepiness?)" , "Brief bowing gesture" , "Signify assent" , "Appear to fall asleep" , "Cain was banished to the land of ..." .)