Extract energy within the law (6)
I believe the answer is:
elicit
'extract' is the definition.
(I know that extract can be written as elicit)
'energy within the law' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'within the law' becomes 'licit' (I've seen this before).
'e'+'licit'='ELICIT'
(Other definitions for elicit that I've seen before include "'Draw out, evoke (6)'" , "Get out" , "Evoke (a response)" , "Draw out, information or explanation" , "Draw out from, invoke" .)