Prime US prosecutor occupies two years going after ambassador (6)
I believe the answer is:
heyday
'prime' is the definition.
(I know that prime can be written as heyday)
'us prosecutor occupies two years going after ambassador' is the wordplay.
'us prosecutor' becomes 'DA' (district attorney).
'occupies' indicates putting letters inside (inserted letters occupy or enter the word).
'two years' becomes 'yy' (an abbreviation for year written twice. I am not sure about the 'two' bit.).
'going after' means one lot of letters go next to another.
'ambassador' becomes 'HE' (His or Her Excellency).
'da' inserted within 'yy' is 'yday'.
'yday' after 'he' is 'HEYDAY'.
Can you help me to learn more?
(Other definitions for heyday that I've seen before include "Period of one's greatest success" , "Period of greatest vigour" , "One's peak period" , "Period of one's greatest productivity" , "Prime time?" .)