The answers to the following clues will have something in common.

1. Like an insincere compliment
2. Relive one's youth
3. Play second fiddle
4. Desperate
5. Shocked
6. Of low priority
7. Mean, hurtful talk about another
8. Part of a kitchen countertop
9. Betrayed by a friend
10. Reject and abandon a friend
11. Regress
12. Strong, sudden, negative reaction
13. Construction site staple
14. Revise or retract an earlier position
15. Keep a protective eye on someone
16. Consecutive, as athletic champioinships
17. Support a losing candidate
18. Unable to function due to illness or injury

Answers:

1. Backhanded
2. Turn back the clock
3. Take a back seat to
4. With your back to the wall
5. Taken aback
6. On the back burner
7. Backbiting
8. Backsplash
9. Stabbed in the back
10. Turn your back on
11. Backslide
12. Backlash
13. Backhoe
14. Backpedal
15. Have your back
16. Back to back
17. Back the wrong horse 
18. Flat on your back