Yes, STARTER is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, A 1, R 1, T 1, E 1, R 1.

Is it a valid word?

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

an electric motor for starting an engine
a contestant in a team sport who is in the game at the beginning
the official who signals the beginning of a race or competition
any new participant in some activity
food or drink to stimulate the appetite (usually served before a meal or as the first course)
a hand tool consisting of a rotating shaft with parallel handle
a culture containing yeast or bacteria that is used to start the process of fermentation or souring in making butter or cheese or dough

Also: starter motor, starting motor.

Not playable in Scrabble US: FLEDGELING.