A world of energy in factory? (6)
I believe the answer is:
planet
'a world' is the definition.
(a planet is a world)
'energy in factory?' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' indicates putting letters inside.
'factory?' becomes 'plant' (a manufacturing plant).
'e' placed within 'plant' is 'PLANET'.
'of' acts as a link.
(Other definitions for planet that I've seen before include "One orbiting star" , "body in orbit" , "Eg, Mars, Venus" , "Mercury?" , "Jupiter is one" .)