Fed up with border guard (6)
I believe the answer is:
defend
'guard' is the definition.
(defend can mean to guard or protect)
'fed up with border' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'with' says to put letters next to each other.
'border' becomes 'end' (I've seen this in other clues).
'fed' backwards is 'def'.
'def'+'end'='DEFEND'
(Other definitions for defend that I've seen before include "Fortify" , "Champion" , "Resist an attack" , "Block, as in cricket" , "Help or protect against attack" .)