The slammer (6)
I believe the answer is:
prison
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">the slammer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for prison that I've seen before include "Place of confinement" , "Gaol" , "Penitentiary" , "Stir - can" , "Cooler" .)