Mercury, say, element at outset brought into factory (6)
I believe the answer is:
planet
'mercury say' is the definition.
(mercury is a kind of planet)
'element at outset brought into factory' is the wordplay.
'at outset' says to take the initial letters.
'brought into' means one lot of letters goes inside another.
'factory' becomes 'plant' (I've seen this before).
The first letter of 'element' is 'e'.
'e' put into 'plant' is 'PLANET'.
(Other definitions for planet that I've seen before include "One orbiting star" , "Earth perhaps" , "massive revolver" , "Large satellite of star" , "Mars or Uranus" .)