Period of one's greatest productivity (6)
I believe the answer is:
heyday
'period of one's greatest productivity' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for heyday that I've seen before include "Highest period in someone's career" , "golden age" , "Period of greatest success and popularity" , "Period of greatest vigour" , "Prime time?" .)