Agree to deny defence (3)
I believe the answer is:
nod
'agree to' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for nod that I've seen before include "Signal assent" , "Signal with the head" , "Move head in assent" , "even Homer sometimes does" , "Signify assent" .)