Cat first found by English river thrown back (6)
I believe the answer is:
feline
'cat' is the definition.
(thesaurus)
'first found by english river thrown back' is the wordplay.
'first' says to take the initial letters.
'by' says to put letters next to each other.
'english' becomes 'e' (abbreviation).
'river' becomes 'nile' (Nile is an example**).
'thrown back' is a reversal indicator.
The initial letter of 'found' is 'f'.
'e'+'nile'='enile'
'enile' back-to-front is 'eline'.
'f'+'eline'='FELINE'
(Other definitions for feline that I've seen before include "Like a cat" , "Like a leopard, say" , "Relating to cats" , "Scots dish" , "Cat-like" .)