Beaten by a masterly fellow (5)

I believe the answer is:

mated

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'beaten' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

'by a masterly fellow' is the wordplay.
'by' says to put letters next to each other.
'masterly' becomes 'm' (this might be a standard abbreviation which I don't know about).
'fellow' becomes 'ted' (man's name).
'a' after 'm' is 'ma'.
'ma'+'ted'='MATED'

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

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