Yes, REDUCER is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, D 2, U 1, C 3, E 1, R 1.

Is it a valid word?

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

a substance capable of bringing about the reduction of another substance as it itself is oxidized; used in photography to lessen the density of a negative or print by oxidizing some of the loose silver
pipefitting that joins two pipes of different diameter

Also: reducing agent.