Partial
The identity of $ X $ is
Metal
Solutions to the cryptic clues:
1D. Deadly $X$ (5)
FATAL(?) - deadly, is also the name of several metal bands
2D. White heroine or cocaine (4)
SNOW - Snow White, and “snow” is slang for cocaine.
3D. Introduction of good old $X$ (4)
GOLD - metal, G(ood) + OLD
4A. $X$ in screwy nail is $X$ object (5)
ANVIL - metal object, (NAIL)* around V (vanadium, a metal)
5A. $X$s scrapped behind engine driver's head for $X$ group (9)
MOTORHEAD(?) (suggested by Rémi Henry) - metal group, MOTOR (engine driver) + HEAD, not sure about “Metals scrapped” though
6A. Unopened salad central solely to leaders of $X$ equation (3'1 3)
???’? LAW - Some kind of law relating to metals - maybe LEN’S LAW, or OHM’S LAW (suggested by ZanyG)? Though not sure how either fit the cryptic clue... We do have SLAW = “salad,” but the rest I have no idea