There are a few basic guidelines to follow when writing a definition:
Wiktionary is a dictionary, not an encyclopedia. A definition does not need to contain every piece of information known about a subject. Rather, it should contain information about the word and what the word refers to, and enough explanation to allow a user to distinguish that word from most other words.
Don't include usage notes in a definition. If additional clarification is needed to separate a term from other words with a similar meaning, then Usage notes can be added.
Keep it simple
Write with simple grammatical structures rather than complicated ones. Limit the use of dependent clauses and phrases, whether set off by commas (or parenthetical). If many such additional modifications are needed in a definition, then that definition probably needs a thorough rewrite.
Place key terms and short explanations near the start of the definition. If a definition requires qualifications and modifying phrases, then try to place those near the end of the definition, after the main point has been made.
Avoid bombast, and get to the point. Padding an entry with superfluous verbiage may sound impressive, but that's not the purpose of a definition.
Avoid complicated terms
Avoid terms that are more complicated or more technical than the term being defined. The purpose of a definition is to clarify meaning, not to obfuscate it.
Use mainstream English words in explanations whenever possible. The more widespread a term is, the more users will benefit. Terms with restricted usage may leave a significant fraction of Wiktionary's readership clueless.
Avoid jargon. Highly technical terms will probably require a user to look up many terms in the definition just to understand what it says. Although there are cases which require the use of a technical term in a specialized field, these should be kept to a minimum.
Avoid words whose use is restricted to certain geographic regions, social or ethnic groups, or that might otherwise leave many readers confused.
It is poor form to define a word with that same word in the definition. Use different terms.
Avoid defining a term solely using etymologically or morphologically related terms. Adverbs are especially prone to being defined by the adjective from which they derive. While it is possible to define words this way, definitions ideally should stand on their own.
Check the primary words in the definition to see whether any point right back at the term being defined. If two entries are defined solely or primarily as each other, then there will be no basis for a reader to understand either definition.