can't use CAST in Query Table?

can't use CAST in Query Table?

I have data broken into half hourly increments.  I'm trying to build a query table that sums the data into each full hour, but I need to display that hour in a time format, ie hh:mm:ss.
To do this I'm trying to use CAST (DATEPART(hour, "DATE")  AS varchar(2)) + ':00' AS Hour,
but I get this response " Unsupported function used in the query.  "

Is there any way around this?