Reverse of determination not half shown by this failure (5)
I believe the answer is:
loser
'this failure' is the definition.
('loser' can be a synonym of 'failure')
'reverse of determination not half' is the wordplay.
'reverse of' shows that the letters should be reversed in order.
'determination' becomes 'resolution' (resolution is a kind of determination).
'not half' means to take half the letters.
'resolution' halved is 'resol'.
'resol' reversed gives 'LOSER'.
'shown by' acts as a link.
(Other definitions for loser that I've seen before include "sad type?" , "One failing regularly" , "lemon" , "Someone who fails consistently" , "Person who's a failure" .)