def clean_word(word: str) -> str:
    """Return all alphanumeric characters from word, in the same order as
    they appear in word, converted to lowercase.

    >>> clean_word('')
    ''
    >>> clean_word('AlreadyClean?')
    'alreadyclean'
    >>> clean_word('very123mes$_sy?')
    'very123messy'

    """

    cleaned_word = ''
    for char in word.lower():
        if char.isalnum():
            cleaned_word = cleaned_word + char
    return cleaned_word