Breeding ground (6)
I believe the answer is:
hotbed
'breeding ground' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)