Rats covered in coal dust occasionally (4)
I believe the answer is:
cads
'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" .)