Source of nutrient under salt lake (4)
I believe the answer is:
tarn
'lake' is the definition.
(tarn is a kind of lake)
'source of nutrient under salt' is the wordplay.
'source of' suggests taking the first letters.
'under' is a charade indicator (letters next to each other).
'salt' becomes 'tar' ('tar' can be a synonym of 'salt').
The first letter of 'nutrient' is 'n'.
'n' after 'tar' is 'TARN'.
(Other definitions for tarn that I've seen before include "Small mountain lake" , "High water" , "Scottish mountain lake" , "Mountain pool: French department" .)