Yes, CASTANETS is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, S 1, T 1, A 1, N 1, E 1, T 1, S 1.

Is it a valid word?

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

a percussion instrument consisting of a pair of hollow pieces of wood or bone (usually held between the thumb and fingers) that are made to click together (as by Spanish dancers) in rhythm with the dance

Also: finger cymbals.