These are the lowest limits of zone covering roads banning a diesel's lead (5)
I believe the answer is:
zeros
'roads banning a diesel's lead' is the definition.
I can't tell whether this definition defines the answer.
'these are the lowest limits of zone covering' is the wordplay.
'these are the lowest limits of' becomes 'eros' (I can't explain this - if you can you should believe this answer much more).
'zone' becomes 'z'.
'covering' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'eros' after 'z' is 'ZEROS'.
Can you help me to learn more?
(Other definitions for zeros that I've seen before include "ducks" , "loves" , "zips" , "nothings" , "Noughts" .)