Greatest work (6,4)
I believe the answer is:
magnum opus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greatest work</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 magnum opus that I've seen before include "Greatest work of an artist" , "major achievement" , "Great work" , "Creative feat" , "one's best" .)