Yes, flat is a Scrabble word!

Adverb

1. With flat sails

2. In a forthright manner; candidly or frankly

Adjective

1. Having a surface without slope, tilt in which no part is higher or lower than another

2. Having a relatively broad surface in relation to depth or thickness

3. Not modified or restricted by reservations

4. Stretched out and lying at full length along the ground

5. Lacking contrast or shading between tones

6. (of a musical note) lowered in pitch by one chromatic semitone

7. Flattened laterally along the whole length (e.g., certain leafstalks or flatfishes)

8. Lacking taste or flavor or tang

9. Lacking stimulating characteristics; uninteresting

10. Having lost effervescence

11. Sounded or spoken in a tone unvarying in pitch

12. Horizontally level

13. Lacking the expected range or depth; not designed to give an illusion or depth

14. Not reflecting light; not glossy

15. Commercially inactive

Noun

1. A level tract of land

2. A shallow box in which seedlings are started

3. A musical notation indicating one half step lower than the note named

4. Freight car without permanent sides or roof

5. A deflated pneumatic tire

6. Scenery consisting of a wooden frame covered with painted canvas; part of a stage setting

7. A suite of rooms usually on one floor of an apartment house