They don't believe he requires capital in Greece (8)
I believe the answer is:
heathens
'they don't believe' is the definition.
(I've seen this before)
'he requires capital in greece' is the wordplay.
'requires' is a charade indicator (letters next to each other).
'capital in greece' becomes 'athens' (I can't explain this - if you can you should give a lot more credence to this answer).
'he'+'athens'='HEATHENS'
Can you help me to learn more?
(Other definitions for heathens that I've seen before include "Those who do not believe in the main religions" , "They don't believe in any of the major religions" , "pagans" , "Athens he searched for non-believers" .)