Sailor joins navy in body of water (4)
I believe the answer is:
tarn
'body of water' is the definition.
(tarn is a kind of body of water)
'sailor joins navy' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor).
'joins' is a charade indicator (letters next to each other).
'navy' becomes 'n' (common abbreviation eg in RN Royal Navy).
'tar'+'n'='TARN'
'in' is the link.
(Other definitions for tarn that I've seen before include "Mountain pool: French department" , "Scottish mountain lake" , "Small mountain lake" , "High water" .)