Length of time (6)
I believe the answer is:
period
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">length of time</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 period that I've seen before include "Span of time" , "Stretch - cycle" , "Stretch of time" , "Phase" , "Stop in America" .)