Mum keeps gold fish in tomb (9)
I believe the answer is:
mausoleum
'tomb' is the definition.
(I know that mausoleum is a type of sepulchre)
'mum keeps gold fish' is the wordplay.
'keeps' is an insertion indicator.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'fish' becomes 'sole' (sole is a kind of fish).
'au'+'sole'='ausole'
'mum' placed around 'ausole' is 'MAUSOLEUM'.
'in' acts as a link.
(Other definitions for mausoleum that I've seen before include "Building with a tomb" , "One may lie impressively in this" , "Wonder of World tomb" , "Magnificent tomb or monument" , "Sepulchre" .)