If you turn on the option to require approval for list items (under library settings, versioning settings, top option), then you can reject items that require clarification with the comment "Clarify" or some such. On the "My Submissions" view that is created when you turn on approval, users can see their item as rejected and can see the comment. They can then modify the item, which will change its status back to "pending."
None of this will automatically alert the user that their item was rejected for clarification. You may, however, be able to set up a SharePoint workflow that could do that. I don't have an offhand description of how that would work, but perhaps someone else here has a suggestion. You could also go to SharePoint forums like stackexchange for further advice.