ept - If inept means incompetent, then ept, as its opposite, means competent and therefore expert.
ear, eye, egg. There's a wonderful book called the dictionary which has every word in the English language listed alphabetically. It makes it wonderfully easy to find such words.
PRO
ACE
ace
PRO
Dan
Dan
Mogul.
One seven letter word, One two letter word and a five letter word, One three letter word and a four letter word, One three letter word and two two letter words, Two three letter words, Three two letter words, A two letter word and a four letter word, A six letter word, A two letter word and a three letter word, A five letter word, Two two letter words, A four letter word, A three letter word, or A two letter word.
professional
the words for food expert are gourmet, epicure, gastronome.
Authority can be used as another word for expert and starts with an "A". Example: An authority on the subject of crime, notes that...
I've is a three letter word. I'll is another three letter word.
Imp is a three-letter word for rapscallion.
A three letter word for debt is owe.