The answers to the first three clues below have something in common that is revealed by the fourth clue.

1. Sewers' convenience
2. System of inter-connected people or things
3. Road that loops around a city to ease traffic
4. Prudent motto - often appearing in work places

Answers:

1. Pin cushion
2. Network
3. Beltway
4. Safety first (Safety pin, Safety net, Safety belt)

Credit: Jerry E. Rosman