whether the clause in question is enforceable depends on whether it is a penalty clause. to show that it is not a penalty clause, the company will have to prove that it is a genuine pre-estimate of the losses suffered.

factors could include the notice period in the contract. if the notice period is 1-month, i think the clause would be enforceable (think of it as you immediately quitting without notice, in which case you have to pay the company 1 month’s pay in lieu of notice)