In 4E, teleports require line of sight, but do not require line of effect. You could teleport through a glass window, no problem.
So, in general, if a power gives you line of sight where otherwise you would not have it, the answer is "yes, you can teleport there". That is provided the distance teleported from your location does not exceed normal restrictions. The distance teleported is based on your physical location, not your viewpoint. So you cannot combine the powers to travel further than normal. But you could use the combination to teleport through walls and around corners.
Regarding "Seer's Clairvoyance", it does appear to give you line of sight from a separate location, it is a Close Burst 20 power. This means:
The power combination definitely allows you to teleport around corners, or into locations hidden from your physical position.
Close Burst 20 is not affected by line-of-sight issues, just line-of-effect. So that does allow you to get a vision located on the other side of a wall of fog, or to get past an illusionary wall, or if you are engulfed by a darkness effect. However, the Seer's Clairvoyance is not limited by standard Close Burst 20 rules . . .
The wording of Seer's Clairvoyance specifically mentions the ability to get a viewpoint anywhere within 20 squares, ignoring line of effect. Therefore, the combination with Fey Step would allow you see into blocked off areas and teleport through 4 squares of solid rock, into any space big enough to contain you. You could bypass quite a few barriers this way, including gaining access to the inside of a locked, windowless room. This seems a bit powerful for Heroic Tier power combination, but is IMO the correct RAW reading of this power.
Note it's a Daily power, and you cannot change the location once set. That makes a teleport into a locked, windowless room a potential one-way trip, or maybe a self-imposed prison for 4 hours or more.
Your teleport range is not extended. You may still only Fey Step for 5 squares as standard.