That monstrous loch (4)
I believe the answer is:
ness
'that monstrous loch' is the definition.
(as in the Loch Ness Monster)
This is all the clue.
(Other definitions for ness that I've seen before include "something added for quality" , "Headland, promontory" , "Scottish loch - headland" , "Scottish lake with traditional monster" , "Scottish Lough of monster myth" .)