Yes, SECRETARIAT is a Scrabble word!

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

Is it a valid word?

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

an administrative unit responsible for maintaining records and other secretarial duties; especially for international organizations
thoroughbred that won the triple crown in 1973

Not playable in Scrabble US: SECRETARIATE.