One of the Greens possesses place in old European capital (6)
I believe the answer is:
peseta
'one of the greens possesses' is the definition.
I can't tell whether this defines the answer.
'place in old european capital' is the wordplay.
'place' becomes 'set' ('set' can be a synonym of 'place').
'in' means one lot of letters goes inside another.
'old european capital' becomes 'pea' (I can't justify this - if you can you should give a lot more credence to this answer).
'set' going within 'pea' is 'PESETA'.
Can you help me to learn more?
(Other definitions for peseta that I've seen before include "Old tender" , "Old bit" , "Obsolete coin" , "little cash is required" , "Spanish bread once" .)