Built in Functions use with recordsets

Hi.

I'm trying to get an average value (avg()) of a certain attribute in a
table. The snag is, I don't want this across a whole table. I only want it
over a certain set of records. In effect, I want to do something like:

SELECT somefield
FROM sometable
WHERE otherfield = 'criteria'
ORDER BY thirdfield
LIMIT 10

and then do an avg(somefield).

Can this be done without using temp tables, in a single query? I tried
making a function that does this, but even if I used temp tables, the
function didn't work, claiming that the table didn't exist.

Ideally, I want to do something like:

SELECT avg
(
    SELECT somefield
    FROM sometable
    WHERE otherfield = 'criteria'
    ORDER BY thirdfield
    LIMIT 10
) as somefieldname

But the parser doesn't seem to like it.

How can I do this?

Thanks.

Gordan