Celtic’s work in the transfer window has not even begun yet but already the club are set to land a tidy financial windfall. Players will always come and go at the club but what Celtic have been extremely good at over the last decade or so is that when our top players leave, the club ensures they protect their investment by inserting a future sell-on clause. Celtic are likely to benefit when players like Jeremie Frimpong and, before Arsenal set a price tag of £10m, Kieran Tierney move on to their next ventures. We have also seen in the past where a sell-on clause inserted in Liverpool’s Virgil …