Organ very loud? Musicians’ Union installed protection for listener (7)
I believe the answer is:
earmuff
'protection for listener' is the definition.
'organ very loud? musicians' union installed' is the wordplay.
'organ' becomes 'ear' (ear is a kind of organ).
'very loud?' becomes 'ff' (musical abbreviation).
'musicians' union' becomes 'mu'.
'installed' means one lot of letters goes inside another.
'ear'+'ff'='earff'
'earff' going around 'mu' is 'EARMUFF'.
(Other definitions for earmuff that I've seen before include "to provide restriction on listeners" , "Warmer worn on head" , "Pad to protect part of head from cold" , "(One of two) protecting against cold or noise in the hearing organ ..." , "Pad to keep lug warm" .)