Sailor on northern lake (4)
I believe the answer is:
tarn
'lake' is the definition.
(I know that tarn is a type of lake)
'sailor on northern' is the wordplay.
'sailor' becomes 'tar' (tar can mean a sailor).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'tar'+'n'='TARN'
(Other definitions for tarn that I've seen before include "Small mountain lake" , "Scottish mountain lake" , "Mountain pool: French department" , "High water" .)