Slit (4)
I believe the answer is:
slot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">slit</span>' is the definition.<br/ ><span class="explanation-format__explanation">(slot is a kind of slit)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for slot that I've seen before include "Long narrow opening" , "Lots (anag.)" , "Scheduled time -- aperture for coins" , "Space for coin or programme" , "Space for inserting coin" .)