Vendor, more flexible accommodating one (8)
I believe the answer is:
supplier
'vendor' is the definition.
(I know that supplier is a type of vendor)
'more flexible accommodating one' is the wordplay.
'more flexible' becomes 'suppler' (I've seen this before).
'accommodating' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'suppler' enclosing 'i' is 'SUPPLIER'.
(Other definitions for supplier that I've seen before include "One who provides" , "Wholesaler, e.g." , "Provider of a product" , "One providing something" , "Provider (of goods)" .)