Going north, pass over end of rich expanse of water (4)
I believe the answer is:
loch
'expanse of water' is the definition.
'loch' can be an answer for 'water' (a loch is a body of water). I am not sure about the 'expanse of' bit.
'going north pass over end of rich' is the wordplay.
'going north' shows that the letters should be reversed in order.
'pass' becomes 'col' (col is a term for a mountain pass).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'end of' indicates one should take the final letters.
The final letter of 'rich' is 'h'.
'col' written backwards gives 'loc'.
'loc'+'h'='LOCH'
Can you help me to learn more?
(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" , "Scottish word for lake" .)