Hubbub is one problem (5)
I believe the answer is:
noise
'hubbub' is the definition.
(I've seen this before)
'is one problem' is the wordplay.
'problem' is an anagram indicator.
'is'+'one'='isone'
'isone' is an anagram of 'NOISE'.
(Other definitions for noise that I've seen before include "Unwanted sounds" , "Row" , "Hubbub" , "Kind of pollution" , "Confused sounds" .)