The most common and most likely answer to this crossword clue is the 8 letter word BULLHEAD
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Buddy explains!

The straight part
We know that "mud cat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mud cat = bullhead.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 BULLHEAD mean?
BULLHEAD noun- Any of a variety of related species of generally dark-colored catfish in the family Ictaluridae.
- (Europe, Asia) Any of various sculpins of the suborder Scorpaenoidei
- (Europe, Asia) The European bullhead, Cottus gobio.
- A fish of species Gobiomorphus gobioides.
- An obstinate person.
- A bullhead rail.
- A chronograph (watch or stopwatch) with two push buttons arranged like the horns of a bull on the top end of the case, typically with the crown between them.