AERATOR is a 7 letter word that starts with A.

1. \A"["e]r*a`tor\, n.
   That which supplies with air; esp. an apparatus used for
   charging mineral waters with gas and in making soda water.
   

2. \A"["e]r*a`tor\, n.
   That which supplies with air or gas; specif.:
   (a) An apparatus used for charging mineral waters with gas
       and in making soda water.
   (b) A fumigator used to bleach grain, destroying fungi and
       insects.