The Clarendon is only for international students. Most of your questions can be answered in a quick Googling--see, for example, the the Clarendon FAQ: http://www.admin.ox.ac.uk/io/funding/cl ... 2008-9.pdf. This does not say what proportion of the awards were for full stipend, tuition, and fees; but overall last year there were 104 awards (4.6% of applicants).
I don't know whether they notify people all at once or in batches, but in 2006 I heard back on 31 March. I think I know of someone in Classics on a Clarendon, so I don't think they discriminate against impractical fields.