Treat successfully (4)
I believe the answer is:
cure
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">treat successfully</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cure that I've seen before include "Parish priest in France" , "Preserve by smoking - make better" , "restore one to health" , "Restorative" , "Remedy or preserve" .)