Sailor close to mountain lake ... (4)
I believe the answer is:
tarn
'lake' is the definition.
(tarn is a kind of lake)
'sailor close to mountain' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor).
'close to' indicates one should take the final letters (the close/ending of).
The last letter of 'mountain' is 'n'.
'tar'+'n'='TARN'
(Other definitions for tarn that I've seen before include "Mountain pool: French department" , "High water" , "Scottish mountain lake" , "Small mountain lake" .)