Question : Calculated Field

I want to create a field in a table such that...

the value of that field is constructed from a portion of another field in the same record.  The constructed value should always be automatically updated and stored back
into the table when the value in the other field changes.

I do not want to do any of this via a form - but rather through the underlying table.

I have tried to set the default value of the field but
access rejected my reference to the other field in the
record (maybe my syntax was incorrect? I don't think so).

Is there any way to do thsi table-based calculated field?

Answer : Calculated Field

Take time to think about the answers already given.  

1) you can not set the default value based on a calculation of other fields.

2) it is not good database design to store calculations in the table.

3) it is not good database design to combine fields together to make one field.

You need to really, really, think about your database design.