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' ('scar' can be a synonym of 'mark').
'has' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'returned' is a reversal indicator.
'tim' written backwards gives 'mit'.
'i'+'mit'='imit'
'scar' enclosing '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" .)