Installed in house, a reed organ (3)
I believe the answer is:
ear
'organ' is the definition.
(ear is a kind of organ)
'installed in house a reed' is the wordplay.
'installed in' indicates the answer is hidden within the clue.
'EAR' is hidden in the letters of 'house a reed'.
(Other definitions for ear that I've seen before include "Organ of hearing" , "Sense organ" , "one hears" , "Organ inside heart" , "Go astray" .)