Definition of Sell-on Clause
Not every footballer is a one-club player. Inevitably, players will want to move on to a higher or lower-tier club as their career progresses. Clubs usually go along with this, as they don't want to risk losing promising or star players for free. However, they still want to make as much money off of a departing asset as possible. Enter the sell-on clause.

Some of the more recent high-profile sell-on clauses amongst Europe's top 5 leagues include Angel di Maria's move to PSG (giving £5 million to former club Real Madrid), Raheem Sterling to Manchester City (giving £8 million to Queen's Park Rangers) and Cesc Fabergas (from Barcelona to Chelsea, giving Arsenal £5.6 million). Of course, clubs can't see into the future and might miss out on one of these chances. One of the most high-profile cases of missing a potentially profitable sell-on clause was when Southampton waived the sell-on clause for Gareth Bale to Tottenham, who would then go on to move to Real Madrid for £86 million.
The sell-on clause can make quite a lot of money if used on the correct players. Knowing which players those are can make the difference between a club surviving or even thriving.