Oily gross piece of cake (6)
I believe the answer is:
greasy
'oily' is the definition.
(synonyms)
'gross piece of cake' is the wordplay.
'gross' becomes 'gr' (abbreviation).
'piece of cake' becomes 'easy' (I've seen this in another clue).
'gr'+'easy'='GREASY'
(Other definitions for greasy that I've seen before include "well-lubricated" , "Thickly oleaginous" , "'Fatty, oily (6)'" , "Sycophantic" , "Sticky with fat" .)