Give guns to old Spaniard in the battlefield (10)
I believe the answer is:
armageddon
'battlefield' is the definition.
(I know that Armageddon is a battlefield)
'give guns to old spaniard' is the wordplay.
'give guns to' becomes 'arm' (to arm is to equip with weapons).
'old' becomes 'aged' (similar in meaning).
'spaniard' becomes 'don' (term for a Spanish gentleman).
'arm'+'aged'+'don'='ARMAGEDDON'
'in the' is the link.
(Other definitions for armageddon that I've seen before include "final battle" , "Catastrophic conflict" , "bloody battle" , "Last war's ending" , "Biblical battle" .)