Yes, SUBSUMPTION is a Scrabble word!

Letter values in Scrabble US: S 1, U 1, B 3, S 1, U 1, M 3, P 3, T 1, I 1, O 1, N 1.

Is it a valid word?

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

the premise of a syllogism that contains the minor term (which is the subject of the conclusion)
incorporating something under a more general category

Also: minor premise, minor premiss.