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