You can't. Only those values that are also allowed for untyped constants (ordinals, floating point values, strings, sets) are allowed as default values.
You need to solve this using an overload:
procedure Proc(aCl: Cl);
begin
end;
procedure Proc; inline;
begin
Proc(C1);
end;
If you place the implementation of the overload without the parameter after the one with the parameter and the former has the
inline modifier (like in the example above; for a unit you need to add the modifier to the declaration in the
interface section) then you'll essentially have the same as only one call will be involved then.