A family of Microsoft relational database management systems designed for ease of use.
In a table datasheet, I don't think there's any way you can do this. Are you really letting your users enter data directly in table datasheets? I'm guessing, since you say one of the fields is locked, that this is actually happening on a form, not on a table datasheet.
If your users are actually doing this on a form, even in datasheet view, then I can imagine a way that *might* work -- I haven't tested it. You could set the form's KeyPreview property to True, and use the KeyDown event to test whether the key combination that was pressed is Ctrl+V or Shift+Insert. If so, then you could unlock the field in question. Then in the form's Current event you could lock the field again.
Maybe that would work, though only on a form, not on a table datasheet. However, it would not capture other methods whereby the user could insert a record from the clipboard; not even their use of Copy, Paste Append from the ribbon. On a form, it would be better to have a command button to clone the current record. Then the code behind that button would take care of it all:
RunCommand acCmdSelectRecord
RunCommand acCmdCopy
Me.MyLockedField.Locked = False
Runcommand acCmdPasteAppend
Me.MyLockedField.Locked = True