Killer in Windsor Castle (4)
I believe the answer is:
orca
'killer' is the definition.
(synonyms)
'in windsor castle' is the wordplay.
'in' says the answer is hidden in the clue.
'ORCA' is hidden in the letters of 'windsor castle'.
(Other definitions for orca that I've seen before include "Large predator" , "Sea creature" , "ship" , "Killer in deep" , "Massive killer" .)