Transact SQL :: Function To Return Comma Delimited Values From Table Columns

I've to write a function to return a comma delimited values from a table columns

If a table has Tab1 ( Col1,Col2,Col3).

E.g. as below ( the columnName content I want to use as columns for my pivot table

CREATE FUNCTION [RPT].[GetListOfCol]
(
@vCat NVARCHAR(4000)
)
RETURNS @PList
AS
BEGIN
SELECT @PList += N', [' + [ColumnName] +']'
FROM [ETL].[TableDef]
WHERE [IsActive] = 1
AND [Category] = @vCat
RETURN;
END;

I want out put to be as below, I am getting this output from the select query independently by declaring @Plist variable and passing @vcat value, now I want it to be returned from a function when called from a select query output ,Colum1,column2,....