Clue: Handy warning to drivers arriving at Calais? (6)
Answer: ADROIT
Explanation: Definition: Handy
**À DROIT**e [?] would be French for 'to the right' or 'on the right', and drivers arriving at Calais would have to switch to the right side of the road
 not sure if I'm forgetting my French and 'à droit' can mean something similar