Slick piece of cake with great topping (6)
I believe the answer is:
greasy
'slick' is the definition.
(thesaurus)
'piece of cake with great topping' is the wordplay.
'piece of cake' becomes 'easy' (I've seen this in another clue).
'with great' becomes 'gr' (abbreviation used in place names. I am not sure about the 'with' bit.).
'topping' means one lot of letters go next to another.
'easy' put after 'gr' is 'GREASY'.
Can you help me to learn more?
(Other definitions for greasy that I've seen before include "Unctuous or ingratiating" , "Like some hair" , "Covered with fat" , "Like the pole that is hard to climb" , "Oily in manner, covered with lubricant" .)