River always supplying fish (3)
I believe the answer is:
ray
'fish' is the definition.
(ray is a kind of fish)
'river always' is the wordplay.
'river' becomes 'r'.
'always' becomes 'ay'.
'r'+'ay'='RAY'
'supplying' acts as a link.
(Other definitions for ray that I've seen before include "Man who engaged in surrealist photography" , "Flash < Movie Player" , "bit of sunshine" , "Shaft, beam" , "Shaft of light" .)