Tote rag used as a killing instrument (7)
I believe the answer is:
garotte
'a killing instrument' is the definition.
'garotte' can be an answer for 'instrument' (garotte is a kind of instrument). I am not sure about the 'a killing' bit.
'tote rag used' is the wordplay.
'used' indicates an anagram.
'tote'+'rag'='toterag'
'toterag' is an anagram of 'GAROTTE'.
'as' is the link.
Can you help me to learn more?
(Other definitions for garotte that I've seen before include "Strangle" , "Facility to dispose of person" , "Kill by strangulation" , "Spanish method of execution" , "throttle" .)