Unknown sour concoction! Not mine! (5)
I believe the answer is:
yours
'not mine' is the definition.
(I've seen this in another clue)
'unknown sour concoction' is the wordplay.
'unknown' becomes 'y' (algebra).
'concoction' indicates an anagram.
'sour' anagrammed gives 'ours'.
'y'+'ours'='YOURS'
(Other definitions for yours that I've seen before include "Word found before "sincerely" or "faithfully" in a letter" , "Belonging to you" , "Letter sign-off" , "ie Not mine" , "Family, perhaps" .)