Idiosyncrasy, characteristic (11)
I believe the answer is:
peculiarity
I believe this clue is a double definition.
'idiosyncrasy' is the first definition.
(thesaurus)
'characteristic' is the second definition.
(I know that peculiarity is a type of characteristic)
(Other definitions for peculiarity that I've seen before include "Unusual characteristic" , "Unusual feature or habit" .)