River finished with a hundred-fold increase in content (4)
I believe the answer is:
oder
'river' is the definition.
(Oder River is an example)
'in content' is the wordplay.
'in' becomes 'modern' (I've seen this before).
'content' means to look at the middle letters.
The middle of 'modern' is 'ODER'.
'finished with a hundred-fold increase' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for oder that I've seen before include "Polish/German river" , "River forming northern part of the German-Polish border" , "Rode around the European river" , "river, one in central Europe" , "East European river" .)