The most common and most likely answer to this crossword clue is the 9 letter word WALLOWING
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More clues leading to the result WALLOWING
Buddy explains!

The straight part
We know that "swinish behaviour" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swinish behaviour = wallowing.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 WALLOWING mean?
WALLOWING verb- To roll oneself about in something dirty, for example in mud.
- To move lazily or heavily in any medium.
- To immerse oneself in, to occupy oneself with, metaphorically.
- To live or exist in filth or in a sickening manner.
- To wither; to fade.
- The act of one who wallows.