Dead-end streets copper classed ill-designed (3-2-4)
I believe the answer is:
cul-de-sacs
'dead-end streets' is the definition.
('cul-de-sac' can be a synonym of 'dead-end street')
'copper classed ill-designed' is the wordplay.
I cannot really see how this works, but
'copper' could be 'cu' (Cu is the chemical symbol for copper) and 'cu' is found in the answer.
an anagram of 'classed' is 'ldesacs' which is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for cul-de-sacs that I've seen is " Streets with only one way in or out".)