Aquatic mammal showing excessive energy beside river (5)
I believe the answer is:
otter
'aquatic mammal' is the definition.
(otters live by water)
'excessive energy beside river' is the wordplay.
'excessive' becomes 'OTT' (short for 'over the top').
'energy' becomes 'e' (physics symbol as in E=mc2).
'beside' says to put letters next to each other (I've seen this in other clues).
'river' becomes 'r' (abbreviation for river).
'ott'+'e'+'r'='OTTER'
'showing' is the link.
(Other definitions for otter that I've seen before include "Carnivorous aquatic mammal" , "Playful aquatic animal of 'Tarka' fame" , "Aquatic beast" , "Mammal, 'Tarka'" , "furry animal" .)