Listen and learn about harnessing energy (4,2,3)
I believe the answer is:
lend an ear
'listen' is the definition.
(I've seen this in another clue)
'and learn about harnessing energy' is the wordplay.
'about' indicates anagramming the letters.
'harnessing' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'and'+'learn'='andlearn'
'andlearn' anagrammed gives 'lndanear'.
'lndanear' going around 'e' is 'LEND-AN-EAR'.
(Other definitions for lend an ear that I've seen before include "Listen attentively (to)" , "Pay attention" , "Hark" , "Follow Mark Antony" , "Listen - to borrower" .)