Yes, DEFAMATION is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, F 4, A 1, M 3, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

a false accusation of an offense or a malicious misrepresentation of someone's words or actions
an abusive attack on a person's character or good name

Also: hatchet job.