Equip ancient teacher for final battle (10)
I believe the answer is:
armageddon
'final battle' is the definition.
(I've seen this before)
'equip ancient teacher' is the wordplay.
'equip' becomes 'arm' (I've seen this before).
'ancient' becomes 'aged' (I've seen this before).
'teacher' becomes 'don' (don is a kind of teacher).
'arm'+'aged'+'don'='ARMAGEDDON'
'for' acts as a link.
(Other definitions for armageddon that I've seen before include "extremely destructive conflict" , "End of the world" , "Last war's ending" , "Apocalyptic battle" , "End-of-world battle" .)