Yes, PLUNGER is a Scrabble word!

Letter values in Scrabble US: P 3, L 1, U 1, N 1, G 2, E 1, R 1.

Is it a valid word?

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

someone who risks losses for the possibility of considerable gains
someone who dives (into water)
hand tool consisting of a stick with a rubber suction cup at one end; used to clean clogged drains
mechanical device that has a plunging or thrusting motion

Also: plumber's helper.