Check Cell Upon Selection Allowing For Filtered Data

I have a database of names and titles (acutally: #, last name, full name, title, meeting) and at this point I have a column using the following code.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Cells.Count > 1 Then Exit Sub
If Not Intersect(Target, Range("A5:A355")) Is Nothing Then
Range("A5:A355").Cells.Clear
Target.Font.Name = "Marlett"
If Target = vbNullString Then
Target = "a"
Else
Target = vbNullString
End If
End If
End Sub

everything works like I want, except when I use the auto filter, the "a" won't be cleared from cells that are hidden. I need to make sure only one "a" is populated in this column as that's what I'm using with the vlookup function to key the data for my Character Generator.

Maybe I'm not using the best code, I'm new to this and just cobbled together code from a few different places.

Some quick background to what I'm doing, but you may not need it to help. I'm trying to use Excel to populate some cells that will tell our Chyron Character Generator (for those that don't know what that is, it's the device that puts graphics on screen during news programs, etc...) what to put up for lower third titles (and more) for our meetings we cover.