Clue: Freewheel from 18 to 5 across? (5-2-5)
Answer: COAST-TO-COAST
Explanation: Freewheel from 18 to 5 across? (5-2-5)
A pun on 'coast'; of course, neither PHILADELPHIA ('18') nor SEATTLE ('5 across') is actually on the coast, but near enough – on the Delaware River and Puget Sound respectively.