River source really central (4)
I believe the answer is:
ural
'river' is the definition.
(Russian river)
'source really central' is the wordplay.
'source' becomes 'ur' (I can't explain this - if you can you should give a lot more credence to this answer).
'central' indicates the central letters.
The centre of 'really' is 'al'.
'ur'+'al'='URAL'
Can you help me to learn more?
(Other definitions for ural that I've seen before include "Europe/Asia river" , "Mountain range between Europe and Asia" , "Russian river, rising in the mountains between Europe and Asia" , "The - mountains separate Asia and Europe" , "Russian river giving name to mountains" .)