Standard river fish (4)
I believe the answer is:
parr
'fish' is the definition.
(I've seen this before)
'standard river' is the wordplay.
'standard' becomes 'par' (I've seen this before).
'river' becomes 'r'.
'par'+'r'='PARR'
(Other definitions for parr that I've seen before include "Catherine ---, Henry VIII's widow" , "survivor of the decapitator?" , "Young salmon; Queen Catherine ---" , "Katherine --, last wife of Henry VIII" , "Young salmon with spots" .)