Signify agreement (3)
I believe the answer is:
nod
'signify agreement' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nod that I've seen before include "Show agreement (or sleepiness?)" , "Indicate assent or recognition" , "Brief bowing gesture" , "Assent with head movement (3)" , "may get gesture of approval" .)