Mark has sent back Tim with one sword (8)
I believe the answer is:
scimitar
'sword' is the definition.
(I know that scimitar is a type of cavalry sword)
'mark has sent back tim with one' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'has' indicates putting letters inside.
'sent back' shows that the letters should be reversed in order.
'with' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'tim'+'i'='timi'
'timi' in reverse letter order is 'imit'.
'scar' placed around 'imit' is 'SCIMITAR'.
(Other definitions for scimitar that I've seen before include "Type of sword with curved blade" , "Curved oriental sabre" , "Short curved oriental sword" , "Steel" , "I'm racist, owning an oriental sword" .)