PM's residence (6,3)
I believe the answer is:
number ten
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pm's residence</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 number ten that I've seen before include "Last but one in" , "Stand-off" , "centre of government" , "where the PM can be found?" , "Maybe X" .)