Prosperity supplies a barrier to restrict disease (9)
I believe the answer is:
affluence
'prosperity' is the definition.
(I've seen this before)
'a barrier to restrict disease' is the wordplay.
'barrier' becomes 'fence' (fence is a kind of barrier).
'to restrict' is an insertion indicator.
'disease' becomes 'flu' (flu is a kind of disease).
'fence' placed around 'flu' is 'ffluence'.
'a'+'ffluence'='AFFLUENCE'
'supplies' is the link.
(Other definitions for affluence that I've seen before include "Plenty" , "Prosperity or wealth" , "Abundant wealth" , "Fortune" , "being loaded" .)