Loch of the monster (4)
I believe the answer is:
ness
'loch of the monster' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ness that I've seen before include "head" , "Headland, cape" , "Scottish Lough of monster myth" , "Scottish lake with traditional monster" , "Scottish Lough containing legendary monster" .)