Mormon's back next to Salt Lake (4)
I believe the answer is:
tarn
'lake' is the definition.
(I know that tarn is a type of lake)
'mormon's back next to salt' is the wordplay.
'back' suggests the final letters.
'next to' means one lot of letters go next to another.
'salt' becomes 'tar' (both can informally mean a sailor).
The last letter of 'mormon' is 'n'.
'n' after 'tar' is 'TARN'.
(Other definitions for tarn that I've seen before include "Mountain pool: French department" , "Scottish mountain lake" , "Small mountain lake" , "High water" .)