Pit passage (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pit passage</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 adit that I've seen before include "Horizontal mine passage" , "Taking passage" , "Almost horizontal mine shaft" , "Horizontal mine drainage shaft" , "Means of access to mine" .)