Birde böyle deneyin. sehirAd As String, cboSehir As MSForms.ComboBox bunları ekledim ve uyarladım abey.

Sub IlceAktar()

    Dim x As Integer, bul As Range, sehirAd As String, cboSehir As MSForms.ComboBox

    Set cboSehir = Me.ComboBox_Sehir

    Me.ComboBox_Ilce.Clear

    If cboSehir = "" Then GoTo son

    With Sheets("TANIMLAR")

        Set bul = .Range("B:B").Find(cboSehir.Value, , , 1)

        If Not bul Is Nothing Then

            sehirAd = .Cells(bul.Row, 3).Value

            cboSehir.Value = sehirAd

            For x = 2 To .Range("A1000").End(xlUp).Row

                If .Range("A" & x).Value = bul.Value Then _

                        Me.ComboBox_Ilce.AddItem (.Range("D" & x).Value)

            Next

        End If

    End With

son:

    Set bul = Nothing: Set cboSehir = Nothing

End Sub