Goad for a dunderhead (4)
I believe the answer is:
prod
'goad' is the definition.
('prod' can be a synonym of 'goad')
'for a dunderhead' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'a dunderhead' becomes 'd' (head letter of 'dunder').
'pro'+'d'='PROD'
(Other definitions for prod that I've seen before include "Poke; reminder" , "Give a reminder" , "Drop (anag.)" , "Poke (with a finger, etc)" , "Poke or spur" .)