With which the head expresses assent (3)
I believe the answer is:
nod
'assent' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for nod that I've seen before include "Show assent" , "Assenting head gesture" , "Gesture of acceptance" , "Cain left Eden for the Land of . . ." , "slight bow" .)