Fish? Sarah's caught any number by river (4,3)
I believe the answer is:
sand eel
'fish?' is the definition.
(sand eel is a kind of fish)
'sarah's caught any number by river' is the wordplay.
'sarah' becomes 'Sal' (Sally is a common pet form of Sarah).
'caught' indicates putting letters inside.
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'by' is a charade indicator (letters next to each other).
'river' becomes 'dee' (River Dee).
'n'+'dee'='ndee'
'sal' enclosing 'ndee' is 'SAND-EEL'.
(Other definitions for sand eel that I've seen before include "swimmer" , "Shallow-water fish" , "Very small fish that burrows into beaches" .)