The most fitting answer for a crossword clue "formal piece of writing" is "LITERARY" with 8 letters. It directly relates to formal writing, especially in the context of literature and artistic written works.

Other potential answers based on various crossword sources include "ESSAY," "DOCUMENT," and "PROSE," though "literary" matches the clue's emphasis on formality and style in written works. Would you like help with more crossword clues or related writing terminology?