#1
I have a problem that I have not encountered or heard of before so any insight would be useful.

Holding down the high e string on the 7th fret on my guitar seems to have this odd muting like effect. The string just does not vibrate properly and there's no sustain. I think the best way to describe it is that the string vibrates a minimal amount and the note sounds impotent! This is odd because it's only the 7th fret on this string. All other frets on the string and all other strings with this fret are fine.

I don't know whether I just need to clean my neck or anything like that, any constructive input would be appreciated.

The guitar in question is an Ibanez RG8. I last changed strings a couple of months ago. I've had it for a year now and it hasn't given me this trouble before.
"Beer is proof that God loves us and wants us to be happy." -Benjamin Franklin
#2
Could be a little warping in the neck, if it's just on a single fret on one string then it shouldn't be that bad. Take it to a guitar shop and ask them to look at it, they'll probably just need to adjust the truss rod.
#3
If its just one fret, then its not warped and truss rod adjustments wont fix it. warping affects a significant section, usually more than a 1/4 of the neck.

Is there buzzing, like is the string hitting the 8th fret at all? it will be more noticeable when you pluck the string hard.

Try tuning the guitar a whole note low on all strings. And a half note high, (or a whole note if you're brave) If the problem moves to a different fret, then you got a harmonic sink, which is where the wood sucks at a specific note.
#4
doesn't sound like a truss rod job.

either raise the bridge a little for that string or have a tech look at it with a view to dressing the frets.
I've been imitated so well I've heard people copy my mistakes.
- Jimi Hendrix