Member, old fellow, in catastrophic conflict (10)
I believe the answer is:
armageddon
'catastrophic conflict' is the definition.
(I've seen this before)
'member old fellow' is the wordplay.
'member' becomes 'arm' (arm is a kind of member).
'old' becomes 'aged' ('aged' can be similar in meaning to 'old').
'fellow' becomes 'don' (I've seen this before).
'arm'+'aged'+'don'='ARMAGEDDON'
'in' acts as a link.
(Other definitions for armageddon that I've seen before include "End-of-world battle" , "final battle" , "bloody battle" , "Ultimate scrap" , "Biblical battle" .)