Brilliant doubling of six by dunderhead (5)
I believe the answer is:
vivid
'brilliant' is the definition.
(I know that brilliant can be written as vivid)
'doubling of six by dunderhead' is the wordplay.
'doubling of six' becomes 'vivi' (I can't explain this - if you can you should believe this answer much more).
'by' says to put letters next to each other.
'dunderhead' becomes 'd' (head letter of 'dunder').
'vivi'+'d'='VIVID'
Can you help me to learn more?
(Other definitions for vivid that I've seen before include "Lifelike, striking" , "Very bright and colourful" , "Intensely striking in colour" , "Strong and bright, colour perhaps" , "Brilliant" .)