Grey animal on river? Seal (6)
I believe the answer is:
assure
'seal' is the definition.
(thesaurus)
'grey animal on river?' is the wordplay.
'grey animal' becomes 'ass' (donkeys are often grey).
'on' means one lot of letters go next to another.
'river?' becomes 'Ure' (river in Yorkshire).
'ass'+'ure'='ASSURE'
(Other definitions for assure that I've seen before include "Convince, guarantee" , "Make (something) certain to happen" , "Cover by life insurance" , "Tell positively" , "Make sure, guarantee" .)