Unique case of atypical individual (5)
I believe the answer is:
alone
'unique' is the definition.
(I've seen this in another clue)
'case of atypical individual' is the wordplay.
'case of' suggests removing the centre (like emptying a case).
'individual' becomes 'one' (an individual is one person).
'atypical' with its centre removed is 'al'.
'al'+'one'='ALONE'
(Other definitions for alone that I've seen before include "Exclusively, by oneself" , "Isolated; exclusively" , "Solo" , "Single" , "without another" .)