Agree with backwards don (3)
I believe the answer is:
nod
'agree with' is the definition.
(thesaurus)
'backwards don' is the wordplay.
'backwards' is a reversal indicator.
'don' back-to-front is 'NOD'.
(Other definitions for nod that I've seen before include "Where Cain went to" , "Bob the head" , "Gesture of approval with head" , "Signify agreement" , "Assenting head gesture" .)