The dream of more than 500 is disunity (8)
I believe the answer is:
division
'disunity' is the definition.
(thesaurus)
'the dream of more than 500' is the wordplay.
I cannot quite understand how this works, but
'dream' could be 'vision' (I've seen this before) and 'vision' is present in the answer.
'500' could be 'd' (Roman numeral for 500) and 'd' is located in the answer.
A single letter 'i' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for division that I've seen before include "between a regiment and a corps?" , "Military grouping" , "some police?" , "The inverse of multiplication" , "Discord, split" .)