Slender, narrow (4)
I believe the answer is:
thin
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">slender</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">narrow</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for thin that I've seen before include "Fine" , "Spare" , "Made of light material" , "(Of smile) weak, forced" , "Unconvincing" .)