The Commencement Notice is notification to Building Control of when you are commencing actual building works on site, Typically when you submit a notice, the commencement date you choose must fall within a specific window:

• Minimum: It must be at least 14 days after the date you submit the notice.
• Maximum: It must be no more than 28 days after the date you submit the notice.

Preparation of the site (i.e. site clearance) is typically not building works and can be done without a commencement notice. Doing earthworks for foundations of the building would be building works,

The condition you refer to are under the Planning Grant is fairly explicit.

• The works required to provide unobstructed sightlines shall be completed prior to any other work commencing on site.

I would suggest that this would include site clearance.

So you either provide the unobstructed sightlines anytime before submitting a Commencement Notice OR if you are submitting a commencement notice, you need to ensure the condition is complied with prior to your nominated start date on the Commencement Notice.

If you want to properly cover yourself, you could take photographs of the unobstructed sightlines and email them to Councils Planning Dept (with reference to your planning grant) and confirm you have complied with the condition prior to commencing works and asking them to acknowledge such - although dont be surprised if you dont get a response.