Yes, sign is playable
...and is worth 7 points.
Found in the following dictionaries
Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)
"Sign" Definition
noun
A body manifestation that serves to indicate the presence of malfunction or disease.
A conventional figure or device that stands for a word, phrase, or operation; a symbol, as in mathematics or in musical notation.
A displayed structure bearing lettering or symbols, used to identify or advertise a place of business: a motel with a flashing neon sign outside.
A portentous incident or event; a presage: took the eclipse as a sign from God.
A posted notice bearing a designation, direction, or command: an EXIT sign above a door; a traffic sign.
phrasal-verb
Sign in To record the arrival of another or oneself by signing a register.
Sign off Informal To express approval formally or conclusively: got the Congress to sign off on the tax proposal.
Sign off To announce the end of a communication; conclude.
Sign off To stop transmission after identifying the broadcasting station.
Sign on Informal To enlist oneself, especially as an employee: "Retired politicians often sign on with top-dollar law firms” ( New York Times).
verb-transitive
To approve or ratify (a document) by affixing a signature, seal, or other mark: sign a bill into law.
To communicate with a sign or signs: signed his approval with a nod.
To consecrate with the sign of the cross.
To express (a word or thought, for example) by sign language: signed her reply to the question.
To hire or engage by obtaining a signature on a contract: signed a rookie pitcher for next season; sign up actors for a tour.
verb-intransitive
To make a sign or signs; signal.
verb
To place a sign on or mark by .
To represent or indicate by a sign.
To affix a to ratify or attest by hand or seal .
To assign or convey formally .
To write down (one's name).