You need to have a generic WRITE(FORMATTED) binding, bound to a specific procedure that has suitable characteristics. See section 9.6.4.8 in the F2008 standard for more information.
type :: A
real :: value
integer :: descriptor
contains
procedure :: writetype
generic :: write(formatted) => writetype
end type A
...
subroutine writetype(dtv, unit, iotype, v_list, iostat, iomsg)
! Argument names here from the std, but you can name them differently.
class(A), intent(in) :: dtv ! Object to write.
integer, intent(in) :: unit ! Internal unit to write to.
character(*), intent(in) :: iotype ! LISTDIRECTED or DTxxx
integer, intent(in) :: v_list(:) ! parameters from fmt spec.
integer, intent(out) :: iostat ! non zero on error, etc.
character(*), intent(inout) :: iomsg ! define if iostat non zero.
...
write (unit, "(F9.4,1X,A)", IOSTAT=iostat, IOMSG=iomsg) &
dtv%value, describe_foo(dtv%descriptor)
end subroutine writetype
It is probably also worth noting that you need a compiler that implements this!