Loch with a monster (4)
I believe the answer is:
ness
'loch with a monster' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ness that I've seen before include "Headland, cape" , "Scottish lake with traditional monster" , "Where you're reputed to see monster" , "Eliot -, led effort against Al Capone; Scots loch" , "Scottish Lough containing legendary monster" .)