To a dunderhead, no means yes! (3)
I believe the answer is:
nod
'yes' is the definition.
(thesaurus)
'to a dunderhead no' is the wordplay.
I cannot really see how this works, but
'dunderhead' could be 'd' (head letter of 'dunder') and 'd' is located in the answer.
'no' could be 'o' (resembles 0 - 'no' can mean 'zero') and 'o' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'means' acts as a link.
Can you help me to learn more?
(Other definitions for nod that I've seen before include "Show assent" , "What Homer may sometime do" , "Assenting head gesture" , "'A . . . is as good as a wink to a blind horse'" , "Show recognition" .)