Wrong individual unfortunately sourer outside (9)
I believe the answer is:
erroneous
'wrong' is the definition.
(similar in meaning)
'individual unfortunately sourer outside' is the wordplay.
'individual' becomes 'one' (an individual is one person).
'unfortunately' indicates an anagram (a bad or unfortunate spelling).
'outside' means one lot of letters goes inside another (some letters go outside others).
'sourer' with letters rearranged gives 'errous'.
'one' inserted inside 'errous' is 'ERRONEOUS'.
(Other definitions for erroneous that I've seen before include "Mistaken, in error" , "Incorrect, wrong" , "Mistaken - inaccurate" , "not right" , "with mistakes" .)