Sign of assent (3)
I believe the answer is:
nod
'sign of assent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nod that I've seen before include "Sign of assent or salutation" , "Show recognition" , "'A . . . is as good as a wink to a blind horse'" , "positive sign" , "Land of exile" .)