Possible Clues:

• Flatter setting for third rate flower
• There's 11 across of this in H.M.S. Pinafore
• Dockside seller is a weedy type
• Bishop, say, presented with prize flower
• Pressure after British complete copper plant
• Bloomer getting prize at dairy show
• Common grassland plant, poisonous to livestock
• Cow, perhaps, a pest on the lawn
• Weed with rich yellow flowers
• Object on front of cottage covered by good climbing plant