read a CLOB value from DB and set in Custom field in JIRA?

[Nic Brough -Adaptavist-]

You'll need to code to connect to your other database, read the value and then think about how to store it in JIRA - the only built-in field type that could hold a CLOB is the "free text" field, but it's only really suitable for text, if your CLOB is anything more than text, you'll need to translate it to text to make it useful to the reader.