Subject confuses them about energy (5)
I believe the answer is:
theme
'subject' is the definition.
('theme' can be a synonym of 'subject')
'confuses them about energy' is the wordplay.
'confuses' is an anagram indicator.
'about' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'them' with letters rearranged gives 'thme'.
'thme' enclosing 'e' is 'THEME'.
(Other definitions for theme that I've seen before include "thing under discussion" , "Subject (on which person writes, speaks)" , "[(type of) PARK]" , "Subject (of talk, text, etc)" , "Thread, melody" .)