Period of greatest success (6)
I believe the answer is:
heyday
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">period of greatest success</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for heyday that I've seen before include "One's best time" , "Period of greatest success and popularity" , "The time of greatest success or prosperity" , "Period of one's greatest success" , "Prime time?" .)