London lockup (4)
I believe the answer is:
gaol
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">london lockup</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 gaol that I've seen before include "The Ballad of Reading -- (Wilde)" , "stir" , "Alternative to jail" , "Prison (British spelling)" , "See 1" .)