Mark takes one medium object to be a sword (8)
I believe the answer is:
scimitar
'a sword' is the definition.
(scimitar is a kind of sword)
'mark takes one medium object' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'takes' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'object' becomes 'it' (pronoun for an inanimate object).
'i'+'m'+'it'='imit'
'scar' placed around 'imit' is 'SCIMITAR'.
'to be' acts as a link.
(Other definitions for scimitar that I've seen before include "This cut" , "one that is very sharp" , "Sword with curved blade used by the Turks and Persians" , "Eastern sword" , "Short curved oriental sword" .)