Yes, COMMANDER is a Scrabble word!

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

Is it a valid word?

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

an officer in command of a military unit
someone in an official position of authority who can command or control others
a commissioned naval officer who ranks above a lieutenant commander and below a captain
an officer in the airforce

Also: commanding officer, air force officer.