Nessie originally seen on salt lake (4)
I believe the answer is:
tarn
'lake' is the definition.
(I know that tarn is a type of lake)
'nessie originally seen on salt' is the wordplay.
'originally' indicates taking the first letters.
'seen on' says to put letters next to each other.
'salt' becomes 'tar' (both can informally mean a sailor).
The first letter of 'nessie' is 'n'.
'n' after 'tar' is 'TARN'.
(Other definitions for tarn that I've seen before include "Mountain pool: French department" , "Small mountain lake" , "Scottish mountain lake" , "High water" .)