Yes, MONSTRANCE is a Scrabble word!

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

Is it a valid word?

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

proof by a process of argument or a series of proposition proving an asserted conclusion
(Roman Catholic Church) a vessel (usually of gold or silver) in which the consecrated Host is exposed for adoration