Breeding ground (6)
I believe the answer is:
hotbed
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">breeding ground</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 hotbed that I've seen before include "Breeding ground" , "breeding-ground for problems" , "Place with ideal conditions for rapid growth" , "Place with ideal conditions for development" , "Scene of rapid growth, as of intrigue" .)