Champion getting fed up before the finish (6)
I believe the answer is:
defend
'champion' is the definition.
('defend' can be a synonym of 'champion')
'fed up before the finish' is the wordplay.
'fed up' becomes 'def' ('fed' going up in a down clue).
'before' means one lot of letters go next to another.
'the finish' becomes 'end' (I've seen this in other clues).
'def'+'end'='DEFEND'
'getting' acts as a link.
(Other definitions for defend that I've seen before include "Safeguard" , "Protect from harm" , "Stand up for" , "Maintain against attack" , "Protect - endorse" .)