Mormon's end by salt lake (4)
I believe the answer is:
tarn
'lake' is the definition.
(I know that tarn is a type of lake)
'mormon's end by salt' is the wordplay.
'end' suggests the final letters.
'by' is a charade indicator (letters next to each other).
'salt' becomes 'tar' (both can informally mean a sailor).
The final letter of 'mormon' is 'n'.
'n' put 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" .)