Are we talking about .ac (Ascension Island) or .ac.uk (UK academic institutions)?
If it's the latter then it comes under Nominet (and it's a restricted SLD). If it's the former then (surprise surprise) it comes under the Ascension Island ccTLD registry whose website is at www.nic.ac.
I'm sure a generic word under .ac would probably be worth something to someone - but that's the same for generic words under any TLD.
I can't imagine there being much interest from UK academic institutions which are all well-established under their ac.uk domains. I certainly wouldn't associate .ac with "academic" - I only associate .ac.uk with "academic" because it's what I grew up with Internet-wise.
Other than for some play on words (like mani.ac) I can't really see much use for .ac outside Ascension Island...