One showing a desire for essentially deadly sadism acts (6)
I believe the answer is:
addict
'one showing a desire' is the definition.
(someone with an addiction)
'essentially deadly sadism acts' is the wordplay.
'essentially' indicates the central letters.
'sadism acts' becomes 'dict' (I can't justify this - if you can you should believe this answer much more).
The middle of 'deadly' is 'ad'.
'ad'+'dict'='ADDICT'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for addict that I've seen before include "One hooked on drugs" , "Person compulsively given to taking drugs" , "Overly-dependent person" , "Someone who's hooked?" , "One unable to kick habit" .)