Tinkers about before heading off to see riverside mammals (6)
I believe the answer is:
otters
'mammals' is the definition.
(otter is a kind of mammal)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for otters that I've seen before include "Freshwater carnivorous mammals" , "Sleek short-furred swimmers" , "Water animals with webbed feet and brown fur" , "Aquatic carnivores" , "Or test out these mammals" .)