ladyalex, I've had time to consider my previous reply to your query, and I now realise that what I wrote, at 13:14, was tripe. This may make more sense:
OVAL = a cricket ground in London.
A cricket team is often referred to as an XI.
'and' in Latin is ET.
If we take 'oval' as XI, then put Latin 'and' outside it, we get E XI T, which is (more or less) 'out'.
In my view, and I'm not saying this just because I had a problem with this particular clue, many of the clues that we are faced with on this site are simply poorly written. In this clue (27a), I think that my reasoning is correct, but the compiler has not hinted at a Latin connection, which is an omission on his/her part. And I say this from the point of view of an ex-crossword puzzle compiler.