Submarine vacated in East River (5)
I believe the answer is:
seine
'river' is the definition.
(Seine is an example)
'submarine vacated in east' is the wordplay.
'vacated' means to remove the middle letters (internal letters vacate the word).
'east' becomes 'E' (synonyms).
'submarine' with its middle removed is 'se'.
'se'+'in'+'e'='SEINE'
(Other definitions for seine that I've seen before include "French river -- type of fishing net" , "Second longest river in France" , "Paris river" , "Paris's river" , "Parisian river" .)