Is start a word in the Scrabble Dictionary

Yes, start is playable

...and is worth 5 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Start" Definition

noun

A beginning; a commencement.
A part that has become dislocated or loosened.
A place or time of beginning.
A position of advantage over others, as in a race or an endeavor; a lead.
A startled reaction or movement.

verb-intransitive

Sports To be in the initial lineup of a game or race.
To become loosened or disengaged.
To come quickly into view, life, or activity; spring forth.
To have a beginning; commence. See Synonyms at begin.
To move suddenly or involuntarily: started at the loud noise.

verb-transitive

Sports To enter (a participant) into a race or game.
Sports To play in the initial lineup of (a game).
Sports To put (a player) into the initial lineup of a game.
To cause to become displaced or loosened.
To commence; begin.

idiom

Start something Informal To cause trouble.
To start with At the beginning; initially.
To start with In any case.

verb

To move suddenly and violently.
To react with a sudden brief involuntary movement.
To issue with sudden force.
To come into being, activity, or operation.
To protrude or seem to protrude.