CL, as provided by IBM, only supports message subfiles. The PowerCL: CL for Files product allows full support for subfiles. The display file containing the subfile record formats can be declared with either the Declared File using CLF (DCLFCLF) command or the File Description Specification (FILE) command.
(one of the PowerCL: CLF developers)
You can declare a Physical, Logical or Display file in CL/400 using the DCLF statement. You can read a Physical or Logical using the RCVF statement. You can write a Display file record using the SNDF command or read one using the RCVF command, you can write and read a Display file record using the SNDRCVF command.
CL/400 (to my knowledge) does not support subfile display formats. For subfile processing you will need to use a HLL such as RPG or COBOL.