Yes, FOSSA is a Scrabble word!

Letter values in Scrabble US: F 4, O 1, S 1, S 1, A 1.

Is it a valid word?

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

a concavity in a surface (especially an anatomical depression)
monotypic genus of Madagascar civets closely related to palm civets
largest carnivore of Madagascar; intermediate in some respects between cats and civets

Also: genus fossa, fossa cat, cryptoprocta ferox.