Clue: Prime US prosecutor occupies two years going after ambassador (6)
Answer: HEYDAY
Explanation: Definition: Prime
 [ DA(abbrev. for "District Attorney", public prosecutor in the US) **contained in**(occupies) Y,Y(2 x abbrev. "year") ] **placed below**(going after, in a down clue) HE(abbrev. for "His/Her Excellency", form of address for an ambassador).
Defn: …/period of someone's or something's greatest success/at one's peak.