Non-civil English county about to be swept away by lake! (5)
I believe the answer is:
surly
'non-civil' is the definition.
(surly can mean rude or impolite)
I don't understand how the rest of the clue works.
(Other definitions for surly that I've seen before include "Morose" , "Grumpily bad-tempered" , "Ill-tempered, unfriendly" , "Gruff and grumpy" , "Ill-natured" .)