Assuming that you have an entry at X across with no clue and one unchecked letter, the clue for X across will contain a one-word definition of the grid entry, enabling you to determine which of the possibilities is the correct one.
The last 'Wrong Number' comp was 2,305:
http://www.andlit.org.uk/azed/cluelist.php?comp_no=2305
In this instance the entry which needed just a one-word definition was PATCH (asterisked in the puzzle and clued simply as 'Piece', the equivalent of 6dn here), and the word lacking a clue was AGREE (the equivalent of X across here). The list of successful clues may give you some help in terms of what is required clue-wise. As you will see, two successful entries used the same definition as Azed ('piece', the equivalent of 'strumpet' here); this is acceptable but not compulsory.