Clue: Used to lack experience, but following advice to young man? Capital! (6)
Answer: WARSAW
Explanation: Definition: Capital
 **Reversal of**(in an across clue, … following advice to young man, from the phrase "Go west, young man", about America's expansion westward) [ WAS(used to be/applicable previously) RAW(to lack experience/green) ].
Defn: … city.