Low energy following small animal (5)
I believe the answer is:
moose
'animal' is the definition.
(type of animal)
'low energy following small' is the wordplay.
'low' becomes 'moo' (lowing of cattle is their sound).
'energy' becomes 'e' (physics symbol as in E=mc2).
'following' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'e' after 's' is 'se'.
'moo'+'se'='MOOSE'
(Other definitions for moose that I've seen before include "North American deer" , "US deer with enormous antlers" , "Large N American deer like European elk" , "North American elk" , "Large US deer" .)