Aquatic mammals chap found by a river (8)
I believe the answer is:
manatees
'aquatic mammals' is the definition.
(manatee is a kind of aquatic mammal)
'chap found by a river' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'found by' means one lot of letters go next to another.
'river' becomes 'tees' (River Tees ).
'man'+'a'+'tees'='MANATEES'
(Other definitions for manatees that I've seen before include "Water creatures" , "Sirenians" , "Large plant-eating mammals of tropical waters" , "Men at sea (anag) - aquatic mammals" .)