Your clues need to give players an idea of what form the answer takes.
For example, for 16th president, it would be fair to expect the answer to be Abraham Lincoln, so 'nickname' is definitely needed in the clue.
If you were looking for his last name only, then you need to specify that too, either in the clue itself or in the hint.
One mistake that many puzzle writers make has to do with verb tenses, or confusions between verbs and nouns.
They often express the clue in the past tense, 'Raced', for example, but then the answer is present tense [RUN] when, according to the clue it should be [RAN].
Or, the word might be the noun [HIKE] for example, but the clue is phrased in verb form 'Walking in the woods'. The way the clue is stated, the answer should really be [HIKING], not [HIKE].
In this case, the clue should read 'A walk in the woods'.
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Editor: Movies/Celebrities/Crosswords
"To insult someone we call him 'bestial'. For deliberate cruelty and nature, 'human' might be the greater insult." - Isaac Asimov