Think I’m not in favour of energy (7)
I believe the answer is:
imagine
'think' is the definition.
(synonyms)
'i'm not in favour of energy' is the wordplay.
'not in favour of' becomes 'agin' (I've seen this before).
'energy' becomes 'e' (physics symbol as in E=mc2).
'im'+'agin'+'e'='IMAGINE'
(Other definitions for imagine that I've seen before include "Picture, conceive" , "Form a mental picture" , "Fancy - dream" , "1971 John Lennon song" , "A Gemini? (anag)" .)