Is the result of the other query a single value or multiple records?
If there are multiple records, your query to get the sum should use this field expression:
SumOther: (Select Sum(field1) from otherquery)
If you need to restrict which records in otherquery get included, you need to do this:
SumOther: (Select Sum(a.field1) from otherquery as a
Where a.KeyField=thisquery.KeyField)