River forming part of a modern European border (4)
I believe the answer is:
oder
'river' is the definition.
(River Oder in central Europe)
'part of a modern european border' is the wordplay.
'part' indicates taking the first letters.
'a modern' becomes 'd' (I can't explain this - if you can you should give a lot more credence to this answer).
'european' becomes 'Eur' (common abbreviation).
'border' suggests removing the centre (I've seen 'borders of' mean this).
'eur' with its centre taken out is 'er'.
The first letter of 'of' is 'o'.
'o'+'d'+'er'='ODER'
'forming' is the link.
Can you help me to learn more?
(Other definitions for oder that I've seen before include "East European river" , "German river that empties into the Baltic Sea" , "School blazer insignia" , "Rode around the European river" , "German banker" .)