Prosperity: a barrier to limit bug (9)
I believe the answer is:
affluence
'prosperity' is the definition.
(thesaurus)
'a barrier to limit bug' is the wordplay.
'barrier' becomes 'fence' (fence is a kind of barrier**).
'to limit' is an insertion indicator.
'bug' becomes 'flu'.
'fence' enclosing 'flu' is 'ffluence'.
'a'+'ffluence'='AFFLUENCE'
(Other definitions for affluence that I've seen before include "Abundant wealth" , "Fortune" , "Wealth - abundance" , "being loaded" , "Prosperity or wealth" .)