Yes, PRETENDER is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, E 1, T 1, E 1, N 1, D 2, E 1, R 1.

Is it a valid word?

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

a claimant to the throne or to the office of ruler (usually without just title)
a person who makes deceitful pretenses
a person who professes beliefs and opinions that he or she does not hold in order to conceal his or her real feelings or motives

Also: role player.