Mormon's end by salt lake (4)
I believe the answer is:
tarn
'lake' is the definition.
(tarn is a kind of lake)
'mormon's end by salt' is the wordplay.
'end' indicates one should take the final letters.
'by' says to put letters next to each other.
'salt' becomes 'tar' (both can informally mean a sailor).
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" , "Mountain pool: French department" , "Small mountain lake" , "High water" .)