Loch with an elusive monster (4)
I believe the answer is:
ness
'loch with an elusive 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" , "head" , "Loch of monster fame" , "Eliot -, led effort against Al Capone; Scots loch" .)