Mark has one - Tim returned weapon (8)
I believe the answer is:
scimitar
'weapon' is the definition.
(scimitar is a kind of weapon)
'mark has one tim returned' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'has' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'returned' is a reversal indicator.
'tim' written backwards gives 'mit'.
'i'+'mit'='imit'
'scar' going around 'imit' is 'SCIMITAR'.
(Other definitions for scimitar that I've seen before include "Sword with curved blade used by the Turks and Persians" , "Curved oriental sabre" , "Double-edged sword" , "Eastern sword" , "Type of sword with curved blade" .)