"CMP Entity Bean"? Is this a EJB 2.x question or a EJB3/JPA entity question? I'll assume the latter due to this forum being "EJB 3.0".
See entityManager.merge() - it merges an unattached JPA entity bean meaning whatever values are in the bean will be put into the DB. It doesn't merge one particular field - the entire bean is merged. I don't know of a way to force only a single field (dirty or not) to be merged into the DB - I don't think its possible, short of you writing your own JPQL UPDATE query and executing that explicitly.
I slightly misspoke - entityManager.merge() can merge either an attached or unattached JPA entity bean (it doesn't just have to be unattached).