It's crucial to hear a murder suspect (7)
I believe the answer is:
eardrum
'it's crucial to hear' is the definition.
(part of the ear necessary for hearing)
'a murder suspect' is the wordplay.
'suspect' indicates an anagram.
'a'+'murder'='amurder'
'amurder' with letters rearranged gives 'EARDRUM'.
(Other definitions for eardrum that I've seen before include "Ear membrane" , "Vibrating membrane in the head" , "Organ membrane" , "Listener partly" , "Aural membrane" .)