Sexy bottom? It might encourage rapid but unwelcome developments (6)
I believe the answer is:
hotbed
'it might encourage rapid but unwelcome developments' is the definition.
'sexy bottom?' is the wordplay.
'sexy' becomes 'hot' ('hot' can be similar in meaning to 'sexy').
'bottom?' becomes 'bed' ('bed' can be a synonym of 'bottom').
'hot'+'bed'='HOTBED'
(Other definitions for hotbed that I've seen before include "Scene of rapid growth, as of intrigue" , "Breeding ground" , "vicious place" , "Forced to grow up early in this" , "breeding-ground for problems" .)