Monarch raving to vicar about one (8)
I believe the answer is:
victoria
'monarch' is the definition.
(Queen Victoria)
'raving to vicar about one' is the wordplay.
'raving' indicates anagramming the letters.
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'to'+'vicar'='tovicar'
'tovicar' is an anagram of 'victora'.
'victora' placed around 'i' is 'VICTORIA'.
(Other definitions for victoria that I've seen before include "Queenly" , "Queen, Australian state and plum" , "Wood" , "Largest lake in Africa" , "Capital of the Seychelles" .)