Yes, MONOCLONALS is a Scrabble word!

Letter values in Scrabble US: M 3, O 1, N 1, O 1, C 3, L 1, O 1, N 1, A 1, L 1, S 1.

Is it a valid word?

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

any of a class of antibodies produced in the laboratory by a single clone of cells or a cell line and consisting of identical antibody molecules

Also: monoclonal antibody.