Lengthiest (7)
I believe the answer is:
longest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lengthiest</span>' is the definition.<br/ ><span class="explanation-format__explanation">('long' can be a synonym of 'lengthy')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for longest that I've seen before include "Having the most length" , "For the most time" , "Of greatest duration" , "Least short" , "Extending farthest" .)