SQL> create table test71
  2  (a varchar2(2000));

Table created.
SQL> insert into test71
  2  values
  3  ('first line
  4  second line');

1 row created.

SQL> commit;

Commit complete.

SQL> select * from test71;

A
----------------------------------------------------------------------------------------------------
first line
second line

SQL> ed
Wrote file afiedt.buf

  1* select replace(a,chr(10),' ') from test71
SQL> /

REPLACE(A,CHR(10),'')
----------------------------------------------------------------------------------------------------
first line second line