How monitor TEMP space allocation for a particular object type like Clob\Blob
If you are connecting to an Oracle instance 11.2+, you can fetch data from the column temp_space_allocated of the view v$active_session_history, which shows the amount of TEMP memory (in bytes) consumed by your session at the time the sample was taken.

Otherwise, you may look at the V$TEMPSEG_USAGE view, column blocks, and convert it to megabytes depending on your tbs block size.