Possible Clues:

• One side of capital city on river, most significant
• Least superficial
• At its most low, river a nuisance
• Extremely rich Scottish banker — it's annoying
• Booming in the extreme
• Most obscure bug at bottom of river
• Most learned to loathe record for the first time
• Lowest form vermin following actress Sandra
• Scottish, Welsh, English or Irish flower blight -- which is most intense?
• Like the far end of a pool