Guard is fed up with finish (6)
I believe the answer is:
defend
'guard' is the definition.
(defend can mean to guard or protect)
'fed up with finish' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'with' means one lot of letters go next to another.
'finish' becomes 'end' (synonyms).
'fed' in reverse letter order is 'def'.
'def'+'end'='DEFEND'
'is' acts as a link.
(Other definitions for defend that I've seen before include "Act as a guard" , "Fortify" , "Champion" , "Help or protect against attack" , "Represent the accused" .)