Polish trick to restrict current in ancient river (7)
I believe the answer is:
rubicon
'ancient river' is the definition.
'rubicon' can be an answer for 'river' (I've seen this before). I'm not sure about the 'ancient' bit.
'polish trick to restrict current' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'trick' becomes 'con' (I've seen this before).
'to restrict' is an insertion indicator.
'current' becomes 'i' (symbol for electric current).
'rub'+'con'='rubcon'
'rubcon' enclosing 'i' is 'RUBICON'.
'in' is the link.
Can you help me to learn more?
(Other definitions for rubicon that I've seen before include "What's important to cross" , "Caesar crossed this river in an irreversible step" , "River symbolically crossed" , "Its crossing irrevocable" , "Caeser crossed this river in a decisive step- no going back" .)