Solid border on some flags? (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Cryptic definition: “flags” here are flagstones, such as might be found on a pavement, with kerbstones forming the border at the edge. Our US readers would probably call them curbstones, and they’d also find flagstones on a sidewalk instead of a pavement.