Yes, KNOCKER is a Scrabble word!

Letter values in Scrabble US: K 5, N 1, O 1, C 3, K 5, E 1, R 1.

Is it a valid word?

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

(Yiddish) a big shot who knows it and acts that way; a boastful immoderate person
a person who knocks (as seeking to gain admittance)
one who disparages or belittles the worth of something
either of two soft fleshy milk-secreting glandular organs on the chest of a woman
a device (usually metal and ornamental) attached by a hinge to a door

Not playable in Scrabble US: DOORKNOCKER.