Calm when covered by gross oily stuff (6)
I believe the answer is:
grease
'oily stuff' is the definition.
(I've seen this in another clue)
'calm when covered by gross' is the wordplay.
'calm' becomes 'ease' (I've seen this before).
'when covered by' means one lot of letters go next to another (in a down clue, some letters go above others).
'gross' becomes 'gr' (abbreviation).
'ease' after 'gr' is 'GREASE'.
(Other definitions for grease that I've seen before include "Possible cause of slip-up" , "-- gun; -- monkey" , "Oily lubricant" , "Film with Travolta and Newton-John" , "Musical starring John Travolta and Olivia Newton-John" .)