Awfully sloppy replies the start of a dangerous decline (8,5)
I believe the answer is:
slippery slope
'the start of a dangerous decline' is the definition.
'awfully sloppy replies' is the wordplay.
'awfully' is an anagram indicator.
'sloppy'+'replies'='sloppyreplies'
'sloppyreplies' anagrammed gives 'SLIPPERY SLOPE'.
(Other definitions for slippery slope that I've seen before include "Course leading to a bad end" , "Course likely to lead one into trouble" , "Downward-leading course" , "Restoration to life" , "dangerous course" .)