Yes, PROSECUTION is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, O 1, S 1, E 1, C 3, U 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

the institution and conduct of legal proceedings against a defendant for criminal behavior
the lawyers acting for the state to put the case against the defendant
the continuance of something begun with a view to its completion

Also: criminal prosecution.