Sailor heads north for lake (4)
I believe the answer is:
tarn
'lake' is the definition.
(tarn is a kind of lake)
'sailor heads north' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor**).
'heads' indicates taking the first letters.
The initial letter of 'north' is 'n'.
'tar'+'n'='TARN'
'for' acts as a link.
(Other definitions for tarn that I've seen before include "High water" , "Mountain pool: French department" , "Small mountain lake" , "Scottish mountain lake" .)