Angry about a dunderhead (3)
I believe the answer is:
red
'angry' is the definition.
(someone may go red when angry)
'about a dunderhead' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'a dunderhead' becomes 'd' (head letter of 'dunder').
're'+'d'='RED'
(Other definitions for red that I've seen before include "A colourful communist" , "'If you see it, perhaps you're angry (3)'" , "Florid" , "Crimson" , "Merchant flag" .)