It means more to a dead-wrong dunderhead (5)

I believe the answer is:

added

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'it means more' is the definition.
The definition suggests a singular noun but the answer is not.

'dead-wrong dunderhead' is the wordplay.
'wrong' indicates an anagram.
'dunderhead' becomes 'd' (head letter of 'dunder').
'dead' anagrammed gives 'adde'.
'adde'+'d'='ADDED'

'to a' acts as a link.

Can you help me to learn more?

(Other definitions for added that I've seen before include "Extra" , "Said" , "Appended; totalled" , "Summed up" , "Totted" .)

I've seen this clue in the Evening Standard.

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