You don't want to delete
. That deletes rows. You want to update:
update tablename
set description = replace(description, 'float:right;', '')
where description like '% float:right;%';
If you want to delete all rows that match that pattern, then your original query is pretty close:
delete from tablename
where description like '% float:right%';
To see what rows would be affected by either query, use:
select *
from tablename
where description like '% float:right%';