Well it sounds like you are using a dynamic parameter. You have to truncate the field before it gets into crystal for this to work.
Depending on your source and what rights you have you could do this in a Stored proc or a sql view. If you have to do it all in crystal you can use a crystal command. You would need to write the command to pull the 2 fields of data, one for joining to your regular table and the other as the truncated field that you can use as the source for your dynamic parameter.
An example:
select employerid, left(employername,15) as ShortName
from employers
Join this back to the original 'employers' table on the employerid
use the new ShortName field as your paramter source