Crossword Genius

Rats covered in coal dust occasionally (4)

Ross

I believe the answer is:

cads

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'rats covered' is the definition.
I can't judge whether this definition defines the answer.

'coal dust occasionally' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'coal'+'dust'='coaldust'
The alternate letters of 'coaldust' are 'CADS'.

'in' acts as a link.

Can you help me to learn more?

(Other definitions for cads that I've seen before include "'Blackguards, bounders (4)'" , "Rotters" , "Knaves" .)

I've seen this clue in The Irish Times.
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