Oracle SQL Developer 3.1.07 extra spaces between characters using listagg

are you using UTF-16 + NVARCHAR2 by any chance? eg this:

SQL> select * from nls_database_parameters where parameter='NLS_NCHAR_CHARACTERSET';

PARAMETER                      VALUE
------------------------------ ----------------------------------------
NLS_NCHAR_CHARACTERSET         AL16UTF16

SQL> drop table test;

Table dropped.

SQL> create table test(a nvarchar2(10));

Table created.

SQL> insert into test values ('test');

1 row created.

SQL> insert into test values ('test 2');

1 row created.

SQL> select listagg(a, ',') within group (order by 1) from test group by 1;

LISTAGG(A,',')WITHINGROUP(ORDERBY1)
--------------------------------------------------------------------------------
 t e s t, t e s t   2

you could cast to a char to get round this. IF this is not acceptible, you need to raise a ticket with Oracle support.

SQL> select listagg(to_char(a),',') within group (order by 1) from test group by 1;

LISTAGG(TO_CHAR(A),',')WITHINGROUP(ORDERBY1)
--------------------------------------------------------------------------------
test,test 2

SQL>

This is currently a known bug with no fix:

Bug 13501087 11.2.0.3 RDBMS 11.2.0.3 SQL EXECUTION PRODID-5 PORTID-226

Abstract: LISTAGG RETURN STRANGE DATA

*** 12/14/11 05:12 am *** (ADD: Impact/Symptom->WRONG RESULTS )

  SubComponent: SQL Analytics
  ===========================
  DETAILED PROBLEM DESCRIPTION
  ============================
  When using LISTAGG function  with NVARCHAR , data is returned with spaces
  between characters

On Oracle 12c I had the issue to with LISTAGG function. I used ASCIISTR() to get around it:

SELECT LISTAGG(TRIM(ASCIISTR(NVL(Name,''))), ';') 
WITHIN GROUP (ORDER BY NAME) AS Names 

Works fine over here.