The most common and most likely answer to this crossword clue is the 5 letter word NIXER
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Buddy explains!
The straight part
We know that "Paddy’s bit of casual work" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case paddy’s bit of casual work = nixer.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does NIXER mean?
NIXER noun- A job or income which is taken in addition to one's normal employment, generally at evenings or weekends. Originally implied that payment was not declared for taxation, but now refers to any work that is not part of one's regular job.