A cause of growing heatedness (6)
I believe the answer is:
hotbed
This could be a double definition.
'a cause' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'growing heatedness' is the second definition.
The definition suggests a singular noun which matches the answer.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for hotbed that I've seen before include "Place with ideal conditions for rapid growth" , "breeding-ground for problems" , "Place with ideal conditions for development" , "Scene of rapid growth, as of intrigue" , "Forced to grow up early in this" .)