Sailor heading north for mountain lake (4)
I believe the answer is:
tarn
'mountain lake' is the definition.
(I've seen this before)
'sailor heading north' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor**).
'heading north' becomes 'n' (abbreviation).
'tar'+'n'='TARN'
'for' is the link.
(Other definitions for tarn that I've seen before include "Scottish mountain lake" , "Small mountain lake" , "Mountain pool: French department" , "High water" .)