Defeated? No feat or achievement (4)
I believe the answer is:
deed
'achievement' is the definition.
(deed is a kind of achievement)
'defeated? no feat' is the wordplay.
'no' indicates named letters should be taken away.
'defeated' with 'feat' removed is 'DEED'.
'or' is the link.
(Other definitions for deed that I've seen before include "What's done" , "Legal document for transfer of property" , "documentary evidence" , "A conscious action" , "Document showing ownership" .)