Addition

12 clues
Increase
Supplement
Ell, maybe
Home project
Math process
Building wing
Blessed event
Extra inclusion
Builder's afterthought
Subtraction's opposite
Opposite of subtraction
Math exercise with plus signs
Similar answers:
Abbreviation
Abbreviations
Abducktion
Abduction
Aberration
Aberrations
Abjuration
Ablation
Ablution
Abolition
Abolitions
Abomination
Abortion
Abovementioned
Abridgeddiction
Absenceofmotion
Absolution
Absorption
Abstentions
Abstraction