The answers to the clues below will be familiar phrases - with the addition of a single letter.

1. Favorite baseball mitt of a devoted fan
2. Street gibberish
3. Future spouse who lives in a penthouse apartment
4. Arthurian path to salvation
5. Rice thrown at a wedding
6. Homesteading at puberty
7. Fake romance around classic building material

Answers:

The added single letter is G, and it's added at the front of the first word in the phrase.

1. GLOVE OF THE GAME
2. GUTTER NONSENSE
3. GROOM WITH A VIEW
4. GRAIL-ROAD TRACKS 
5. GRAIN SHOWER
6. GLAND RUSH
7. GOLD BRICK

Credit:
Paula Gamache (#1-4)