Mostly unlucky? Republican is an extremist (9)
I believe the answer is:
hardliner
'an extremist' is the definition.
(I know that hardliner is a type of extremist)
'mostly unlucky? republican' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'unlucky?' becomes 'hard lines' (both might be said to express sympathy).
'republican' becomes 'R' (abbreviation in US politics).
'hardlines' with its last letter taken away is 'hardline'.
'hardline'+'r'='HARDLINER'
'is' acts as a link.
(Other definitions for hardliner that I've seen before include "man of principle" , "Uncompromising person" , "One's rigorous in application" , "dogmatic type" , "One committed to an uncompromising policy" .)