Best achievement after upsetting education (6)
I believe the answer is:
defeat
'best' is the definition.
(I've seen this before)
'achievement after upsetting education' is the wordplay.
'achievement' becomes 'feat' (feat is a kind of achievement).
'after' means one lot of letters go next to another.
'upsetting' says the letters should be written backwards.
'education' becomes 'ed' (abbreviation - e.g. sex ed).
'ed' back-to-front is 'de'.
'feat' put after 'de' is 'DEFEAT'.
(Other definitions for defeat that I've seen before include "Beat, win against" , "Unsuccessful ending" , "Vanquish, win against" , "Overthrow" , "Rout" .)