Period of one's greatest productivity (6)
I believe the answer is:
heyday
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">period of one's greatest productivity</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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?" .)