Provider more flexible about one (8)
I believe the answer is:
supplier
'provider' is the definition.
(synonyms)
'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' going around '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)" .)