My content type uses an unlimited number of images as "screenshot". I would like to be able to manage the FIRST screenshot of the set as the "cover" of the content type, so to define a "field_cover" and telling Drupal "The value for this field is the first value of field_screenshot". In this way I could reference field_cover in my views for some special cases (like showing the latest mynodes in home page, using the "cover" as the image).
I'm not aware of a "wrapper" field type, but you have a few options.
I've done the same myself in Views by filtering by the "delta" value, which is the individual values in a multivalue field. That way it's easy to require "delta = 0" and get the first value. This has a limitation of currently not working with arguments/contextual filters though.
A second option is the to ctools recently added support for delta values. You can render a field and set both offset, range and reversal of deltas.
Final option I can think of is Computed Field, which could be programmatically coded to act as field wrapper.