Killer whale in Windsor Castle (4)
I believe the answer is:
orca
'killer whale' is the definition.
(synonyms)
'in windsor castle' is the wordplay.
'in' indicates a hidden word.
'ORCA' can be found hidden inside 'windsor castle'.
(Other definitions for orca that I've seen before include "swimmer in black and white" , "27 [WHALE]" , "Cor! a killer whale" , "Killer at sea" , "Killer in deep" .)