Guard fed up before finish (6)
I believe the answer is:
defend
'guard' is the definition.
(defend can mean to guard or protect)
'fed up before finish' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'before' is a charade indicator (letters next to each other).
'finish' becomes 'end' (synonyms).
'fed' reversed gives 'def'.
'def'+'end'='DEFEND'
(Other definitions for defend that I've seen before include "Guard against attack" , "Block, as in cricket" , "Stand up for" , "Resist an attack" , "Compete to retain (sporting title)" .)