Of greatest duration (7)
I believe the answer is:
longest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of greatest duration</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 longest that I've seen before include "For the most time" , "Least short" , "Extending farthest" , "Having the most length" , "Of greatest duration" .)