World's energy consumed by factory (6)
I believe the answer is:
planet
'world's' is the definition.
(I've seen this before)
'energy consumed by factory' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'consumed by' means one lot of letters goes inside another.
'factory' becomes 'plant' (I've seen this before).
'e' inserted into 'plant' is 'PLANET'.
(Other definitions for planet that I've seen before include "Orbiting heavenly body" , "Celestial body orbiting around a star" , "Earth, for example" , "Eg, Mars, Venus" , "Orbiting body in the heavens" .)