Mercury, perhaps — first of elements used in factory (6)
I believe the answer is:
planet
'mercury perhaps' is the definition.
(mercury is a kind of planet)
'first of elements used in factory' is the wordplay.
'first of' says to take the initial letters.
'used in' is an insertion indicator.
'factory' becomes 'plant' (I've seen this before).
The first letter of 'elements' is 'e'.
'e' placed within 'plant' is 'PLANET'.
(Other definitions for planet that I've seen before include "Earth, for instance" , "Celestial body that revolves around the sun" , "Orbiting heavenly body" , "Eg, a ruby" , "World" .)