Animal that lows with energy (5)
I believe the answer is:
moose
'animal' is the definition.
(thesaurus)
'lows with energy' is the wordplay.
'lows' becomes 'moos' ('moo' can be a synonym of 'low').
'with' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'moos'+'e'='MOOSE'
'that' acts as a link.
(Other definitions for moose that I've seen before include "Large northern deer" , "Animal" , "Ungulate" , "Large US deer" , "Large N American deer like European elk" .)