They correspond in worldwide circles (9)
I believe the answer is:
parallels
'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps there's a link between them I don't understand?
'correspond in worldwide circles' is the wordplay.
'correspond' becomes 'parallel' (paralleling is a kind of corresponding).
'in worldwide circles' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'parallel'+'s'='PARALLELS'
Can you help me to learn more?
(Other definitions for parallels that I've seen before include "matches" , "Similarities" , "Imaginary lines around the earth" , "They are always the same distance apart" , "Lines of latitude" .)