Yes, CREDIT is a Scrabble word!

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

Is it a valid word?

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

approval
money available for a client to borrow
an accounting entry acknowledging income or capital items
used in the phrase `to your credit' in order to indicate an achievement deserving praise;
arrangement for deferred payment for goods and services
recognition by a college or university that a course of studies has been successfully completed; typically measured in semester hours
a short note recognizing a source of information or of a quoted passage
an entry on a list of persons who contributed to a film or written work
an estimate, based on previous dealings, of a person's or an organization's ability to fulfill their financial commitments

Also: credit entry, deferred payment, course credit, credit rating.