Frogman in seedy bar by river (5)
I believe the answer is:
diver
'frogman' is the definition.
('diver' can be a synonym of 'frogman')
'seedy bar by river' is the wordplay.
'seedy bar' becomes 'dive' (I've seen this before**).
'by' says to put letters next to each other.
'river' becomes 'r'.
'dive'+'r'='DIVER'
'in' acts as a link.
(Other definitions for diver that I've seen before include "Person operating under water" , "Cheating footballer" , "Bird that plunges" , "One jumping off board" , "One plunges head first into water" .)