Body of water cheers navy (4)
I believe the answer is:
tarn
'body of water' is the definition.
(tarn is a kind of body of water)
'cheers navy' is the wordplay.
'cheers' becomes 'ta' (both can informally mean thank you).
'navy' becomes 'RN' (abbreviation for Royal Navy).
'ta'+'rn'='TARN'
(Other definitions for tarn that I've seen before include "Scottish mountain lake" , "Mountain pool: French department" , "Small mountain lake" , "High water" .)