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' (the ear is an organ).
'an instrument' becomes 'drum' (drum is a kind of instrument).
'ear'+'drum'='EARDRUM'
'that's' is the 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" .)