Help first-class dunderhead (3)
I believe the answer is:
aid
'help' is the definition.
(synonyms)
'first-class dunderhead' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'dunderhead' becomes 'd' (head letter of 'dunder').
'ai'+'d'='AID'
(Other definitions for aid that I've seen before include "Good point" , "welfare" , "Care, assistance" , "Help. Assist" , "grant" .)