Yes, SINGLETON is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, N 1, G 2, L 1, E 1, T 1, O 1, N 1.

Is it a valid word?

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

a single object (as distinguished from a pair)
a set containing a single member
the playing card that is the only card in a suit held in a bridge hand as initially dealt