Unwrinkled
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unwrinkled</span>' is the definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for smooth that I've seen before include "Regular - without problems" , "Calm" , "Flat - consistent - plausible" , "Level, not lumpy" , "Not uneven or rough" .)