Hi thanks for your answer, let me clarify.
Under nornal flanking rules you have to be adjacent to the enemy. Distant Advantage says:
So under this scenario:
A - - - - - - xB
where:
A is an archer
- is a square
x is an enemy
B is an ally of A
does the feat give A Combat Advantage over x?
No, that doesn't work.
The feat does not change how flanking works.
It adds a different way for you to gain combat advantage. Flanking happens to grant combat advantage, too. But it's not the only way.
The feat gives you a way to gain combat advantage. The "trigger" for this happens to occur when someone else is flanking a target. But that doesn't make you "flanking" or changing how flanking works for them or against the target.
If, theoretically, the target would not grant combat advantage against flanking attackers, it would still grant combat advantage to you. (If it would never grant combat advantage, you still wouldn't get it, obviously.)
If the target cannot be flanked, you can never benefit from Distant Avantage from it, since none of your allies can flank it.
The only part that might be unclear is whether you gain the benefits of the feat if you flank the target together with one ally and no other allies flank it. Of course, this is usually irrelevant since flanking already grants you combat advantage, but as mentioned before - theoretically the target might have an ability to negate combat advantage from flanking, in which case the feat might still give you benefits. In that rare case, one would need to rule if it's sufficient if one ally is flanking (with you), or if two allies need to be flanking (but they can be flanking with you, not each other, for large targets), or if two allies together must be flanking regardless of your presence.
Obviously, a rare case and I'd think a DM should be able to find a quick ruling.