dcl-tab-always-indent is a variable defined in `dcl-mode.el'.
Its value is

Controls the operation of the TAB key in DCL mode.
If t, pressing TAB always indents the current line.
If nil, pressing TAB indents the current line if point is at the left margin.
Data lines (i.e. lines not part of a command line or continuation line) are
never indented.

You can customize this variable.