Beaten by one's friend, a dunderhead! (5)

I believe the answer is:

mated

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'beaten by one's' is the definition.
'mated' can be an answer for 'one's' (thesaurus). I'm unsure of the 'beaten by' bit.

'friend a dunderhead' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'a dunderhead' becomes 'd' (head letter of 'dunder').
'mate'+'d'='MATED'

Can you help me to learn more?

(Other definitions for mated that I've seen before include "given match" , "Defeated in game" , "Joined, connected" , "had sex" .)

I've seen this clue in the Evening Standard.

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