Trio not entirely quiet used doorway (9)
I believe the answer is:
threshold
'doorway' is the definition.
(thesaurus)
'trio not entirely quiet used' is the wordplay.
'trio' becomes 'three' ('three' can be a synonym of 'trio').
'not entirely' means to remove the last letter.
'quiet' becomes 'sh'.
'used' becomes 'old' ('old' can be similar in meaning to 'used').
'three' with its last letter removed is 'thre'.
'thre'+'sh'+'old'='THRESHOLD'
(Other definitions for threshold that I've seen before include "Sill of house door" , "Beginning" , "Doorway, entrance" , "Verge" , "gate" .)