Yes, ASCENDER is a Scrabble word!

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

Is it a valid word?

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

someone who ascends
a lowercase letter that has a part extending above other lowercase letters
(printing) the part of tall lowercase letters that extends above the other lowercase letters