Monster of Rowling's creation originally (3)
I believe the answer is:
orc
'monster' is the definition.
(fantasy creature)
'of rowling's creation originally' is the wordplay.
'originally' indicates taking the first letters.
The initial letters of 'of rowling creation' is 'ORC'.
(Other definitions for orc that I've seen before include "Grampus" , "Imaginary creature" , "Killer-whale" , "Fierce goblin" , "Tolkienian creature" .)