Pessimistic sort of believer hiding accomplishment (9)
I believe the answer is:
defeatist
'pessimistic' is the definition.
(I've seen this before)
'sort of believer hiding accomplishment' is the wordplay.
'sort of believer' becomes 'deist' (I've seen this before).
'hiding' indicates putting letters inside.
'accomplishment' becomes 'feat' (feat is a kind of accomplishment).
'deist' placed around 'feat' is 'DEFEATIST'.
(Other definitions for defeatist that I've seen before include "Quitter" , "He's always expecting to be beaten" , "One who always expects failure" , "expecting to fail" , "Person who always expects to lose" .)