Yes, ENTIRELY is a Scrabble word!

Letter values in Scrabble US: E 1, N 1, T 1, I 1, R 1, E 1, L 1, Y 4.

Is it a valid word?

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

to a complete degree or to the full or entire extent (`whole' is often used informally for `wholly')
without any others being included or involved