Animal's excessive energy on runs (5)
I believe the answer is:
otter
'animal's' is the definition.
(thesaurus)
'excessive energy on runs' is the wordplay.
'excessive' becomes 'ott' (short for 'over the top' - informal British).
'energy' becomes 'e' (physics symbol as in E=mc2).
'on' is a charade indicator (letters next to each other).
'runs' becomes 'r' (cricket abbreviation).
'ott'+'e'+'r'='OTTER'
(Other definitions for otter that I've seen before include "Carnivorous aquatic mammal" , "Fish-eater of the weasel family" , "Water mammal" , "carnivore is concerned" , "Aquatic creature" .)