The answer to each clue below is a two-word phrase or compound word. The two words in each phrase or compound word share a certain property. The same shared property is found in each of the answers. What are the answers and what property do they all share?

1. Baseball feat, usually involving first base
2. Riveted look, as in football
3. Period of radioactivity in physics
4. Environmental and asthmatic's concern
5. It may come with a trundle
6. Meal ticket, perhaps
7. First Amendment guarantee
8. Excessive - much more of something than necessary

Answers:

1. Double play
2. Game face
3. Half-life
4. Air quality
5. Daybed
6. Lunch pass 
7. Free press
8. Overkill

Shared property:

Both words in each phrase can be followed by TIME to form a common phrase or compound word:

1. Double time - Play time
2. Game time - Face time
3. Halftime - Lifetime
4. Air time - Quality time
5. Daytime - Bedtime 
6. Lunch time - Pass time
7. Free time - Press time
8. Overtime - Kill time

Credit:
NY Times Crossword
October 8, 2009
Scott Atkinson
TIME AFTER TIME (#1-4)