Yes, RIPOSTE is a Scrabble word!

Letter values in Scrabble US: R 1, I 1, P 3, O 1, S 1, T 1, E 1.

Is it a valid word?

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

a quick reply to a question or remark (especially a witty or critical one)
(fencing) a counterattack made immediately after successfully parrying the opponents lunge

Also: come back.