Old fighter added old weapon (8)
I believe the answer is:
claymore
'old weapon' is the definition.
(historical type of sword)
'old fighter added' is the wordplay.
'old fighter' becomes 'clay' (Muhammad Ali, born Cassius Clay).
'added' becomes 'more' (similar in meaning).
'clay'+'more'='CLAYMORE'
(Other definitions for claymore that I've seen before include "Two-edged broadsword" , "Scots broadsword" , "Something that can cut" , "Scottish Highlanders' broadsword" , "mine, sort of" .)