Of considerable duration (7)
I believe the answer is:
lengthy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of considerable 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 lengthy that I've seen before include "Of considerable duration" , "Rather long" , "Of great duration" , "Protracted" , "Taking up a lot of time" .)