In this case, when your field is Single Select List you need to use the JavaScript custom field to import it as a measure. It is because of the way how the field contains its values and it needs the additional calculation to retrieve this data.
Here is a code for it (replace NNNNN with your customfield ID):
# Select List as measure
[jira.customfield_NNNNN]
data_type = "integer"
measure = true
javascript_code = '''
// create new function which can convert string to number
function setNumber(str)
{
num = Number(str);
if ( Object.prototype.toString.call(num) === "[object Number]" && !isNaN( num )) {
return num;
}
else {
return null;
}
};
// transforms custom field value to string using new function
if(issue.fields.customfield_NNNNN && issue.fields.customfield_NNNNN.value) {
str = issue.fields.customfield_NNNNN.value;
issue.fields.customfield_NNNNN = setNumber(str);
}
'''