A most serene death, using tissue (8,6)
I believe the answer is:
achilles tendon
'tissue' is the definition.
(achilles tendon is a kind of tissue)
'a most serene death using' is the wordplay.
'most serene' becomes 'chillest' (I can't explain this - if you can you should believe this answer much more).
'death' becomes 'end' (synonyms).
'using' becomes 'on' (someone using drugs is 'on' them).
'a'+'chillest'+'end'+'on'='ACHILLES' TENDON'
Can you help me to learn more?
(Other definitions for achilles tendon that I've seen before include "part of leg" , "Tissue" , "part of body" , "accident spot" , "sinew" .)