Not all heard rumours — one's needed to listen! (7)
I believe the answer is:
eardrum
'one's needed to listen' is the definition.
(eardrums are needed for hearing)
'not all heard rumours' is the wordplay.
'not all' says the answer is hidden in the clue.
'EARDRUM' can be found hidden inside 'heard rumours'.
(Other definitions for eardrum that I've seen before include "Ear membrane" , "Auditory membrane" , "Rude ram (anag.)" , "Hearing membrane" , "Organ membrane" .)