Yes, SECONDARIES is a Scrabble word!

Letter values in Scrabble US: S 1, E 1, C 3, O 1, N 1, D 2, A 1, R 1, I 1, E 1, S 1.

Is it a valid word?

SECONDARIES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the defensive football players who line up behind the linemen
coil such that current is induced in it by passing a current through the primary coil

Also: secondary coil, secondary winding.