He does believe about achievement, but he's already lost (9)
I believe the answer is:
defeatist
'he does believe' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'achievement but he's already lost' is the wordplay.
I cannot really understand how this works, but
'achievement' could be 'feat' (feat is a kind of achievement) and 'feat' is located in the answer.
The remaining letters 'deist' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'about' is the link.
Can you help me to learn more?
(Other definitions for defeatist that I've seen before include "Attitude that failure is inevitable" , "Person who always expects to lose" , "Unnecessarily pessimistic" , "One who always expects failure" , "With this attitude, one is resigned to failure" .)