Doubter and believer outside exploit... (9)
I believe the answer is:
defeatist
'doubter' is the definition.
(defeatist is a kind of doubter)
'believer outside exploit' is the wordplay.
'believer' becomes 'deist' (person who believes in a god).
'outside' indicates putting letters inside (some letters go outside others).
'exploit' becomes 'feat' (synonyms).
'deist' going around 'feat' is 'DEFEATIST'.
'and' is the link.
(Other definitions for defeatist that I've seen before include "He's always expecting to be beaten" , "With this attitude, one is resigned to failure" , "Quitter" , "expecting to fail" , "One giving in too easily" .)