Dirty son covered with grease (6)
I believe the answer is:
soiled
'dirty' is the definition.
(soiled can mean dirty or stained)
'son covered with grease' is the wordplay.
I cannot really understand how this works, but
'son' could be 's' (genealogical abbreviation for son) and 's' is found in the answer.
'covered' could be 'oiled' (oiling is a kind of covering) and 'oiled' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for soiled that I've seen before include "Dirty, grimy" , "Dirtied" , "Made dirty" , "Stained and dirty" .)