It may not be a straight forward release clause in the same way it is for a player.
Rather like when Ashworth left for Newcastle, he was placed on gardening leave until Newcastle paid us some compensation. With De Zerbi it coud be that the amount in the contract is what we would accept for us...
In certain ways it's harder to keep a manager than it is a player, but I would expect the club to take the same stance they did with Caicedo.
If Spurs do replace Conte anytime soon then the message would surely be RDZ isn't available because it's not the right time for us.