Doubt one - outside of foreigners - accepted penalties (8)

'doubt' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's a link between them I don't understand?

'one outside of foreigners accepted penalties' is the wordplay.
'one' becomes 'i' (Roman numeral).
'outside of' suggests removing the centre (only letters on the outside of the word).
'accepted' means one lot of letters goes inside another.
'penalties' becomes 'fines' (fine is a kind of penalty).
'foreigners' with its middle taken out is 'fs'.
'fs' placed around 'fines' is 'ffiness'.
'i'+'ffiness'='IFFINESS'

Can you help me to learn more?