Yes, down is playable
...and is worth 9 points.
Found in the following dictionaries
Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)
"Down" Definition
noun
A downward movement; descent.
A soft, silky, or feathery substance, such as the first growth of a human beard.
An expanse of rolling, grassy, treeless upland used for grazing. Often used in the plural.
Any of several breeds of sheep having short wool, originally bred in the Downs of southern England.
Botany A covering of soft, short hairs, as on some leaves or fruit.
preposition
Along the course of: walking down the street.
In a descending direction along, upon, into, or through: rolled down the hill; floating down the river; went down cellar.
In or at: The cans are stored down cellar.
Down along, around, through, toward, in, into, or on .
adverb
Away from the present place.
From a higher to a lower place or position: hiked down from the peak.
From earlier times or people: tradition handed down from one generation to the next.
In or into a sitting, kneeling, or reclining position: knelt down; lying down.
In or into an inactive or inoperative state: The generators went down at midnight.
adjective
Baseball Retired; out: two down in the last of the ninth.
Completed; done: three down, two to go.
Football Not in play: The ball is down on the 50-yard line.
Football Not permitted to advance further in the play because forward progress has stopped, especially by being tackled. Used of a ball carrier.
Learned or known perfectly: had the algebra problems down.
idiom
Down on Informal Hostile or negative toward; ill-disposed to: was down on jogging after his injury.