Yes, HACKER is a Scrabble word!

Letter values in Scrabble US: H 4, A 1, C 3, K 5, E 1, R 1.

Is it a valid word?

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

someone who plays golf poorly
a programmer who breaks into computer systems in order to steal or change or destroy information as a form of cyber-terrorism
a programmer for whom computing is its own reward; may enjoy the challenge of breaking into other computers but does no harm
one who works hard at boring tasks

Also: cyber-terrorist.