Not just smart
I believe the answer is:
ache
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not just smart</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ache that I've seen before include "strong feeling" , "Pine" , "Suffer dull pain" , "Hanker" , "Nagging pain" .)