Drop datafile without dropping a tablespace in Oracle 11g
The Database Backup and Recovery Reference says that you can do a backup by

backup incremental level 0 as compressed backupset database skip inaccessible;


or better

backup incremental level 0 as compressed backupset database skip offline;


But your problem is that the data from datafile 5 is lot if do do not have a valid backup.

If you have a valid backup vou can restore datafile 5.

If you don't have a valid backup you have two option:


you drop the tablespace and everything from this tablespace will be removed forever
export as much as possible , drop and recreate the tablespace and import what you have exported before.


"export as much as possible" means:


you can export the definitions of the objects from this tablespace because these definitions reside in the SYSTEM tablespace.
if the tablespace consists of more than one datafile you can export the data from all remaining datafiles of this taplespace.


These export can be done with the expdp datapump tool.
Use the TABLESPACES parameter to specify the name of the tablespace you want to export. If you only want to export the definitions of the objects set CONTENT=METADATA_ONLY

The import can be done with the impdp tool.