Heads prohibitions about energy (5)
I believe the answer is:
beans
'heads' is the definition.
(bean is a kind of head)
'prohibitions about energy' is the wordplay.
'prohibitions' becomes 'bans' ('ban' can be a synonym of 'prohibition').
'about' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'bans' placed around 'e' is 'BEANS'.
(Other definitions for beans that I've seen before include "nuts" , "Edible kidney-shaped seeds" , "Coffee seeds" , "Haricots for instance" , "Pulses, tomato sauce and grilled bread" .)