Extremely macabre crimes, terribly fascinating (8)
I believe the answer is:
mesmeric
'fascinating' is the definition.
(thesaurus)
'extremely macabre crimes terribly' is the wordplay.
'extremely' means to remove the middle letters.
'terribly' is an anagram indicator.
'macabre' with its centre taken out is 'me'.
'crimes' with letters rearranged gives 'smeric'.
'me'+'smeric'='MESMERIC'
(Other definitions for mesmeric that I've seen before include "Fascinating" , "Hypnotic, spellbinding" , "Hypnotic and compelling" .)