Sword: I’m the very thing to cut lasting mark (8)
I believe the answer is:
scimitar
'sword' is the definition.
(scimitar is a kind of sword)
'i'm the very thing to cut lasting mark' is the wordplay.
'the very thing' becomes 'it' ('that's it!').
'to cut' indicates putting letters inside (some letters cut their way into a word).
'lasting mark' becomes 'scar' (permanent mark of an injury).
'im'+'it'='imit'
'imit' going within 'scar' is 'SCIMITAR'.
(Other definitions for scimitar that I've seen before include "Short curved oriental sword" , "Type of sword with curved blade" , "Eastern sword" , "Curved form of sword" , "This cut" .)