Seasoning on one sausage (9)
I believe the answer is:
pepperoni
'sausage' is the definition.
(I know that pepperoni is a type of sausage)
'seasoning on one' is the wordplay.
'seasoning' becomes 'pepper' (pepper is a kind of seasoning).
'one' becomes 'i' (Roman numeral).
'pepper'+'on'+'i'='PEPPERONI'
(Other definitions for pepperoni that I've seen before include "Spicy salami" , "Spicy Italian sausage" , "Spicy snacks" , "spicy stuff" , "Seasoned beef/pork sausage" .)