Polish in command initially on ancient boundary (7)
I believe the answer is:
rubicon
'ancient boundary' is the definition.
(river famously crossed by Julius Caesar)
'polish in command initially on' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'initially' suggests taking the first letters.
The initial letters of 'in command' is 'ic'.
'rub'+'ic'+'on'='RUBICON'
(Other definitions for rubicon that I've seen before include "Italy/Gaul boundary" , "ancient border" , "River crossed by Caesar in decisive move" , "Caesar's point of no return" , "Cross it to pass the point of no return" .)