Endlessly bad luck, but uncompromising (8)
I believe the answer is:
hardline
'uncompromising' is the definition.
(I've seen this before)
'endlessly bad luck' is the wordplay.
'endlessly' means to remove the last letter.
'bad luck' becomes 'hard lines' (phrase used to express commiserations or sympathy).
'hardlines' with its last letter taken away is 'HARDLINE'.
'but' acts as a link.
(Other definitions for hardline that I've seen before include "uncompromising" , "Tough policy" .)