Host briefly entertaining monster, a sort of accountant (11)
I believe the answer is:
comptroller
'a sort of accountant' is the definition.
(a comptroller oversees accounting)
'host briefly entertaining monster' is the wordplay.
'host' becomes 'compere' (host of an event).
'briefly' means to remove the last letter.
'entertaining' means one lot of letters goes inside another.
'monster' becomes 'troll' (troll is a kind of monster).
'compere' with its final letter removed is 'comper'.
'comper' enclosing 'troll' is 'COMPTROLLER'.
(Other definitions for comptroller that I've seen before include "Financial executive" , "Financial official's title" , "Financial regulator" , "(Generally) financial officer" .)