In an unusual case, you finish with a food additive (5)
I believe the answer is:
sauce
'food additive' is the definition.
(I've seen this in another clue)
'in an unusual case you finish' is the wordplay.
'in' means one lot of letters goes inside another.
'an unusual' indicates an anagram.
'finish' indicates one should take the final letters.
The last letter of 'you' is 'u'.
'case' is an anagram of 'sace'.
'sace' going around 'u' is 'SAUCE'.
'with a' acts as a link.
(Other definitions for sauce that I've seen before include "Dressing served with food" , "Relish or impertinence" , "Meal moistener" , "Informal impudence or food flavouring" , "Lip; foodstuff" .)