Provider more flexible about one (8)
I believe the answer is:
supplier
'provider' is the definition.
(I know that provider can be written as supplier)
'more flexible about one' is the wordplay.
'more flexible' becomes 'suppler' (I've seen this before).
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'suppler' enclosing 'i' is 'SUPPLIER'.
(Other definitions for supplier that I've seen before include "Wholesaler, e.g." , "One providing something" , "Provider of a product" , "One who provides" , "Provider (of goods)" .)