'Python Pandas: How to insert a new column from function of existing column

I have a Pandas dataset which contains a column containing postcode data.

For example "SW12 0ND" is a postcode from a row in the data.

I want to do several things

Insert a new column, populated with values obtained from...
...a function applied to the existing column...
...where the function splits the postcode (string) by space characters and returns the first element

In other words, the new column takes the first part of the postcode: "SW12 0ND" -> "SW12"

python pandas

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