Supports, but fed-up before the finish. Finally quits (7)
I believe the answer is:
defends
'supports' is the definition.
(defending is a kind of supporting)
'fed-up before the finish finally quits' 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 before).
'finally' suggests the final letters.
The final letter of 'quits' is 's'.
'def'+'end'+'s'='DEFENDS'
'but' acts as a link.
(Other definitions for defends that I've seen before include "Supports" , "Protects from attack" .)