You can do this by creating the order-by information on the fly:
SELECT column,
CASE
WHEN column<'c' then 1
ELSE 0
END AS secondary
FROM table
ORDER BY secondary, column;
This will create a fake column called 'secondary' which will contain 1 if the column value is less than 'c', or zero in all other cases.
So 'a' and 'b' will get 1 in the secondary column, and c-z will get zero.
THen if you order by the secondary column first, and the 'real' column second, you'll get all the zeros first and the ones later.