Yes, DECLARERS is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, C 3, L 1, A 1, R 1, E 1, R 1, S 1.

Is it a valid word?

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

the bridge player in contract bridge who wins the bidding and can declare which suit is to be trumps
someone who claims to speak the truth

Not playable in Scrabble US: ASSEVERATOR.