Organ - an instrument that's within hearing (7)
I believe the answer is:
eardrum
'within hearing' is the definition.
I know that 'hearing' relates to this answer. I also know that both the answer and definition are singular nouns
'organ an instrument' is the wordplay.
'organ' becomes 'ear' (ear is a kind of organ).
'an instrument' becomes 'drum' (drum is a kind of instrument).
'ear'+'drum'='EARDRUM'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for eardrum that I've seen before include "Ear membrane" , "Hearing membrane" , "Rude ram (anag.)" , "Otic membrane" , "Auditory membrane" .)