def fetch_field_value(aws_response, field_name):
    """
    Return a value nested within the 'fields' entry of dict *aws_response*.

    The returned value is the second item from a dict with 'key' *field_name*.

    >>> r = {'fields': [{'key': 'someKey', 'stringValue': 'someValue'}]}
    >>> fetch_field_value(r, 'someKey')
    'someValue'
    """
    for container in aws_response['fields']:
        if container['key'] == field_name:
            for k, v in container.items():
                if k != 'key':
                    return v
    raise ValueError('Did not find a field called {0} in response {1}'.
        format(field_name, aws_response))
