Notes achievement first, but he’s already beaten (9)
I believe the answer is:
defeatist
'he's already beaten' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'notes achievement first' is the wordplay.
'notes' becomes 'de' (two musical notes).
'achievement' becomes 'feat' (feat is a kind of achievement).
'first' becomes 'ist' (ist resembles dst).
'de'+'feat'+'ist'='DEFEATIST'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for defeatist that I've seen before include "Person who always expects to lose" , "He's always expecting to be beaten" , "Attitude that failure is inevitable" , "expecting to fail" , "He does not expect to win" .)