The most common and most likely answer to this crossword clue is the 12 letter word MOONLIGHTING
I've seen this crossword clue in The Guardian
M
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Buddy explains!
The straight part
We know that "extra work" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extra work = moonlighting.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 MOONLIGHTING mean?
MOONLIGHTING verb- To work on the side (at a secondary job), often in the evening or during the night.
- (by extension) To engage in an activity other than what one is known for.
- (by extension, of an inanimate object) To perform a secondary function substantially different from its supposed primary function, as in protein moonlighting.
- To carry out undeclared work.
- A period of working on the side (at a secondary job), often in the evening or during the night.