The most common and most likely answer to this crossword clue is the 5 letter word MIRED
I've seen this crossword clue in Independent
M
I
R
E
D
More clues leading to the result MIRED
- Motorway with sign of no movement – one’s got stuck
- Stuck up ladder impatience shows
- Looking angry after busy road gets covered in muck
- Communist after military intelligence, getting bogged down
- Bogged down by Sapper in centre
- Embroiled chief of company controls anger
- Filled with anger, doctor gets stuck in the mud
- Completely stuck: motorway has traffic light
Buddy explains!

The straight part
We know that "covered in muck" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case covered in muck = mired.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 MIRED mean?
MIRED verb- To cause or permit to become stuck in mud; to plunge or fix in mud.
- To sink into mud.
- To weigh down.
- To soil with mud or foul matter.
- A unit of measurement for color temperature.