Yes, LINEBACKER is a Scrabble word!

Letter values in Scrabble US: L 1, I 1, N 1, E 1, B 3, A 1, C 3, K 5, E 1, R 1.

Is it a valid word?

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

a defensive football player who takes a position close behind the linemen
(American football) the position of a defensive football player who plays close behind the line of scrimmage

Also: line backer.