Your question is a bit confusing. So, as much as I understood from your examples, for each sample of *list_asm*, you want to extract the very first word from the string. 

The thing you are doing wrong is treating the string as a list. That is, `['uncomisd xmm2, xmm2', 'jp 0x40', ...]` is considered as a string python python, not a list.

Thus, you need to extract the strings from your list first, then you can't take the first words from all these strings. 

To achieve that, you can use a regular expression to find all the strings that are inside of quotes `'...'`.

    import pandas as pd
    import re
    
    # Read the file into dataframe
    dataFrame = pd.read_json("dataset.json", lines=True)


    # First extract the strings the take the first word of each string  
    dataFrame['opcodes'] = dataFrame['lista_asm'].apply(lambda x: [i.split()[0] for i in re.findall("'([^']*)'", x)])
    print(dataFrame)

or modular form of the code would be:

    import pandas as pd
    import re
    
    # Function to extract the first words from each string
    def extractFirstWord(str):
        listOfWords = re.findall("'([^']*)'", str)
        return [i.split()[0] for i in listOfWords]

    # Read the file into dataframe
    dataFrame = pd.read_json("dataset.json", lines=True)


    dataFrame['opcodes'] = dataFrame['lista_asm'].apply(lambda x: extractFirstWord(x))
    print(dataFrame)

The result: 

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/sZ7oR.png