Energy in deer's legs (6)
I believe the answer is:
stages
'legs' is the definition.
('stage' can be a synonym of 'leg')
'energy in deer's' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' is an insertion indicator.
'deer's' becomes 'stags' (stag is a kind of deer).
'e' put into 'stags' is 'STAGES'.
(Other definitions for stages that I've seen before include "Sagest kind of play venues" , "legs" , "Arranges" , "Positions or points in a process" , "Stations" .)