Outdated fool upset by another fool (4,2)
I believe the answer is:
past it
'outdated fool' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'upset by another fool' is the wordplay.
'upset' says the letters should be written in reverse.
'by another' becomes 'sap' (I am not sure about this - if you are sure you should believe this answer much more).
'fool' becomes 'tit' (I've seen this in other clues).
'sap' backwards is 'pas'.
'pas'+'tit'='PAST IT'
Can you help me to learn more?
(Other definitions for past it that I've seen before include "Spit at, being too old or decrepit" , "Too old to be of use" , "undrinkable?" , "Getting over the hill" , "No longer in one's prime" .)