Agreed silently (6)
I believe the answer is:
nodded
'agreed silently' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nodded that I've seen before include "Signified agreement" , "Signalled agreement" , "Gave silent assent with head movement" , "Assented with head movement" , "Let the head drop" .)