Obtain energy, conserving energy (6)
I believe the answer is:
derive
'obtain' is the definition.
(I know this)
'energy conserving energy' is the wordplay.
'energy' becomes 'drive' (drive can mean energy or enthusiasm).
'conserving' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'drive' placed around 'e' is 'DERIVE'.
(Other definitions for derive that I've seen before include "Originate (from something)" , "Draw" , "Issue" , "Infer; originate (from)" , "Arise (from); deduce" .)