Guard is fed up to death (6)
I believe the answer is:
defend
'guard' is the definition.
(I've seen this before)
'fed up to death' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'to death' 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 "Champion" , "Protect - guard" , "Resist an attack" , "Protect, fight back" , "Protect from harm" .)