CLEAN_AUDIT_TRAIL Runs Slowly ?
If you have never purged the unified audit trails in your database, the first time deletion by CLEAN_AUDIT_TRAIL may take a significant time to complete.
This is because there's no index on column AUD$UNIFIED.EVENT_TIMESTAMP to support the deletion, if you can wait, just be patient.
You may consider to add an index to support the deletion before you do it again. For example:
SQL> create index audsys.aud$unified_idx1 on audsys.aud$unified (dbid, event_timestamp) tablespace sysaux;
Then run DBMS_AUDIT_MGMT.CLEAN_AUDIT_TRAIL to purge audit trails again.