Is it possible to read a CLOB from a remote Oracle database?

The answer is correct in a certain context, for simple select statements over a DB link, you'll get this error:

ORA-22992: cannot use LOB locators selected from remote tables.

From the errors manual:

Cause: A remote LOB column cannot be referenced.
Action: Remove references to LOBs in remote tables.

I also had trouble finding definitive documentation on this...but we just ran into the same issue in our data warehouse. However, there are several work-arounds available, pulling the data over or creating a view for example.

Oracle 12.2 finally added support for distributed LOBs. We can now read data types like CLOB and XMLType over database links without any workarounds.

@Peter Ilfrich: Doesn't that throw an exception when trying to access any clobs over 4000 bytes?

This is a little more convaluted, but it means you can safely pull back small clobs (< 4000) over a dblink.

select dbms_lob.substr@<link>((select <columnName> from dual@<link>), 4000, 1)  
  from <table>@<link>  
 where dbms_lob.getlength@<link>((select <columnName> from dual@<link>)) <= 4000;  

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Reading a CLOB (or a BLOB) over a dblink is possible with this PL/SQL package:
https://github.com/HowdPrescott/Lob_Over_DBLink

If both DB schemes are in the same Oracle instance, you can use the following workaround:

select (select <columnName> from dual) <columnName> from <table>@<link>

This will return you the same as if you would access a local LOB column.