Hearken (6)
I believe the answer is:
listen
'hearken' is the definition.
(I know that hearken is a more specific form of the action listen)
This is all the clue.
(Other definitions for listen that I've seen before include "Follow advice" , "Pay heed (to advice)" , "Give ear (to)" , "Lend an ear (6)" , "Hark!" .)