Yes, FUNDAMENT is a Scrabble word!

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

Is it a valid word?

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

the fundamental assumptions from which something is begun or developed or calculated or explained
the fleshy part of the human body that you sit on
lowest support of a structure

Also: hind end, rear end, tail end.

Not playable in Scrabble US: TOOSHIE, UNDERSTRUCTURE.