RTE doesn't support to select a field in mouse click by default. However, this can be done by run following script. More, to make mouse click to do this, map the mouse click event to this script, and if don't want to show the script status dialog when the script is running, disable "Display Script Status Window" in Options > Script setting.
Set bzhao = CreateObject("BZWhll.WhllObj")
Set fld = bzhao.Field
bzhao.GetCursor row, col
pos = row*80 + col
fld.Pos = pos
startCol = fld.Pos - row * 80
bzhao.SetSelectionStartPos row, startCol
bzhao.ExtendSelection row, startCol + fld.Length
Hope this helps.