A groom follows feline into vault (8)
I believe the answer is:
catacomb
'vault' is the definition.
(I have seen 'underground vault' mean 'catacomb' so perhaps 'vault' could also mean 'catacomb')
'a groom follows feline' is the wordplay.
'groom' becomes 'comb' (combing is a kind of grooming**).
'follows' says to put letters next to each other.
'feline' becomes 'cat' (I've seen this before).
'a'+'comb'='acomb'
'acomb' put after 'cat' is 'CATACOMB'.
'into' acts as a link.
(Other definitions for catacomb that I've seen before include "Underground tunnel for bodies" , "Place to bury" , "Underground burial chamber" , "Subterranean burial place" , "underground vault" .)