I hunt down new type of murder mystery (9)
I believe the answer is:
whodunnit
'type of murder mystery' is the definition.
(I know that whodunnit is a type of murder mystery)
'i hunt down new' is the wordplay.
'new' indicates an anagram.
'i'+'hunt'+'down'='ihuntdown'
'ihuntdown' with letters rearranged gives 'WHODUNNIT'.
(Other definitions for whodunnit that I've seen before include "yarn unravelled finally?" , "book" , "literary mystery" , "Mystery drama" , "crime novel" .)