Mormon's end beside salt lake (4)
I believe the answer is:
tarn
'lake' is the definition.
(tarn is a kind of lake)
'mormon's end beside salt' is the wordplay.
'end' indicates one should take the final letters.
'beside' means one lot of letters go next to another (I've seen this in other clues).
'salt' becomes 'tar' ('tar' can be a synonym of 'salt').
The final letter of 'mormon' is 'n'.
'n' after 'tar' is 'TARN'.
(Other definitions for tarn that I've seen before include "Scottish mountain lake" , "Small mountain lake" , "Mountain pool: French department" , "High water" .)