Give weapons to old fellow for final battle (10)
I believe the answer is:
armageddon
'final battle' is the definition.
(I've seen this before)
'give weapons to old fellow' is the wordplay.
'give weapons to' becomes 'arm' (I've seen this in another clue).
'old' becomes 'aged' (similar in meaning).
'fellow' becomes 'don' (both can mean an academic).
'arm'+'aged'+'don'='ARMAGEDDON'
'for' is the link.
(Other definitions for armageddon that I've seen before include "Final action" , "ultimate conflict" , "extremely destructive conflict" , "Terrible war" , "large-scale fighting" .)