Sailor new on lake (4)
I believe the answer is:
tarn
'lake' is the definition.
(tarn is a kind of lake)
'sailor new on' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor**).
'new' becomes 'n'.
'on' means one lot of letters go next to another.
'tar'+'n'='TARN'
(Other definitions for tarn that I've seen before include "High water" , "Mountain pool: French department" , "Scottish mountain lake" , "Small mountain lake" .)