Young salmon from usual river (4)
I believe the answer is:
parr
'young salmon' is the definition.
(I've seen this before)
'usual river' is the wordplay.
'usual' becomes 'par'.
'river' becomes 'r'.
'par'+'r'='PARR'
'from' is the link.
(Other definitions for parr that I've seen before include "Wife who survived" , "old trout?" , "Catherine --, last wife of Henry VIII" , "Catherine ---, Henry VIII's widow" , "Catherine --, Henry VIII's sixth wife" .)