Urquhart Castle's loch (4)
I believe the answer is:
ness
'urquhart castle's loch' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ness that I've seen before include "Famous loch" , "Loch of monster fame" , "Scottish loch - headland" , "something added for quality" , "Headland, promontory" .)