Champion getting fed up before the finish (6)
'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).
'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".)