Yes, UPSTART is a Scrabble word!

Letter values in Scrabble US: U 1, P 3, S 1, T 1, A 1, R 1, T 1.

Is it a valid word?

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

an arrogant or presumptuous person
a person who has suddenly risen to a higher economic status but has not gained social acceptance of others in that class
a gymnastic exercise performed starting from a position with the legs over the upper body and moving to an erect position by arching the back and swinging the legs out and down while forcing the chest upright

characteristic of someone who has risen economically or socially but lacks the social skills appropriate for this new position

Also: nouveau-riche.