Answer:

ADDN

Answer	Crossword Clue
ADDN	Supplement (abbr.)
ADDN	Bldg. annex
ADDN	Arith. class lesson

ADDN	Annex, for instance: Abbr.
ADDN	Sum: Abbr.
ADDN	Math process: Abbr.
ADDN	Arith. process
ADDN	First-grade subj.
ADDN	Annex: Abbr.
ADDN	Wing: Abbr.
ADDN	Postscript: Abbr.

ADDN	Something appended: Abbr.
ADDN	Annex, briefly
ADDN	Building annex: Abbr.

Crossword

a puzzle consisting of a grid of squares and blanks into which words crossing vertically and horizontally are written according to clues.