Lake of Caledonian highlands primarily (4)
I believe the answer is:
loch
'lake' is the definition.
(loch is a kind of lake)
I don't understand how the remainder of the clue works.
(Other definitions for loch that I've seen before include "Ness or Lomond, for instance" , "A Scottish lake like Lomond" , "Scottish or Irish lake" , "Body of (Scottish?) water" , "A Scottish lake like Ness" .)