Cleric advanced, entering room (4)
I believe the answer is:
dean
'cleric' is the definition.
(I've seen this in other clues)
'advanced entering room' is the wordplay.
'advanced' becomes 'a'.
'entering' means one lot of letters goes inside another.
'room' becomes 'den' (den is a kind of room**).
'a' inserted inside 'den' is 'DEAN'.
(Other definitions for dean that I've seen before include "Kind of clergyman or university official" , "Cathedral cleric" , "A senior clergyman - Swift was one" , "James ..., film actor" , "Clergyman; narrow valley" .)