Narrow fissure or crack (7)
I believe the answer is:
crevice
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">narrow fissure</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">crack</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for crevice that I've seen before include "Crack or fissure." , "Opening" , "Narrow rock fissure" , "Cleft, fissure" , "Split" .)