Long narrow crack (7)
I believe the answer is:
fissure
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">long narrow crack</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 fissure that I've seen before include "Deep crack in rock" , "Long, narrow crack" , "Cleft" , "Split, crack" , "Split - cleavage" .)