Guard is fed up with final item? (8)
I believe the answer is:
defender
'guard' is the definition.
(I've seen this in another clue)
'fed up with final item?' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'with final item?' becomes 'ender' (I can't justify this - if you can you should believe this answer much more).
'fed' in reverse letter order is 'def'.
'def'+'ender'='DEFENDER'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for defender that I've seen before include "One who protects others" , "Protector" , "One who takes up the cause of another" , "One on team" , "One resisting attack" .)