As originally drafted, Clause 9(1) stated:
Before Ministers may use Clause 9, the amendment requires prior enactment of an Act of Parliament approving the final terms of withdrawal.
Clause 9 - on its own wording - can only apply if (as is to be hoped) there is a withdrawal agreement. The clause is about implementation - by Ministers - of the Withdrawal agreement.
The government has already said that there is to be a further Bill to be known as the EU (Withdrawal Agreement and Implementation) Bill - see previous post and Mr Davis' statement to the House of Commons - see Hansard 13th November // Dept. for Exiting the EU announcement.
It is therefore a moot point whether Clause 9 of the present EU (Withdrawal) Bill will ever be required since implementation powers will undoubtedly be included in the proposed bill.
In any event, the present EU (Withdrawal) Bill may yet be further amended.
The amendment is a notable event in the progress of the Bill and, to date, it is the only amendment to have been successful. Nonetheless, it is unlikely to present a serious legal impediment to Brexit and will certainly not prevent it. As suggested on Twitter by Professor Mark Elliott, the amendment may be an incentive for Ministers to achieve a deal which Parliament is likely to approve.
Brexit Time - Parliament takes back control - or does it?
Ministerial Written Statement 13th December (Secretary of State for Exiting the EU). The written statement was not enough to dissuade some MPs voting in favour of the amendment. In the statement David Davis said: "In the UK, the Government have committed to hold a vote on the final deal in Parliament as soon as possible after the negotiations have concluded. This vote will take the form of a resolution in both Houses of Parliament and will cover both the withdrawal agreement and the terms for our future relationship. The Government will not implement any parts of the withdrawal agreement—for example by using clause 9 of the European Union (Withdrawal) Bill—until after this vote has taken place."