Endless haul only found in quiet tomb (9)
I believe the answer is:
mausoleum
'tomb' is the definition.
(mausoleum is a kind of tomb)
'endless haul only found in quiet' is the wordplay.
'endless' indicates the central letters.
'only' becomes 'sole' (synonyms).
'found in' is an insertion indicator.
'quiet' becomes 'mum' (I've seen this before).
The central letters of 'haul' are 'au'.
'au'+'sole'='ausole'
'ausole' inserted into 'mum' is 'MAUSOLEUM'.
(Other definitions for mausoleum that I've seen before include "place such as Taj Mahal" , "Sepulchre" , "Housing for a tomb" , "Wonder of World tomb" , "Magnificent tomb or monument" .)