Incline the head to indicate assent (3)
I believe the answer is:
nod
I believe this is a double definition.
'incline the head' is the first definition.
(I've seen this before)
'indicate assent' is the second definition.
(I've seen this before)
'to' is the link.
(Other definitions for nod that I've seen before include "Acknowledge by gesture" , "Show recognition" , "Indicate assent with head movement" , "Asleep in the land" , "Gesture of acceptance" .)