River important for this animal (6)
I believe the answer is:
donkey
'this animal' is the definition.
(thesaurus)
'river important' is the wordplay.
'river' becomes 'don' (I've seen this before).
'important' becomes 'key' ('key' can be similar in meaning to 'important').
'don'+'key'='DONKEY'
'for' is the link.
(Other definitions for donkey that I've seen before include "Garment associated with building workers" , "Domestic animal" , "Fool" , "Mary's transport" , "Dunce" .)