this works for me....

SQL> set long 9999
SQL> select dbms_metadata.get_ddl('CONSTRAINT','MY_PK') from dual;

DBMS_METADATA.GET_DDL('CONSTRAINT','MY_PK')
--------------------------------------------------------------------------------

ALTER TABLE "SYS"."MY_TEST" ADD CONSTRAINT "MY_PK" PRIMARY KEY ("ID")
USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "SYSTEM" ENABLE