Rich sort of cake popular -- great for starter (7)
I believe the answer is:
rolling
'rich' is the definition.
(rolling in it)
'sort of cake popular great for starter' is the wordplay.
'sort of cake' becomes 'roll' (eg Swiss roll).
'popular' becomes 'in' ('in' can mean trendy).
'for starter' indicates taking the first letters (I've seen 'for starters' mean this).
The initial letter of 'great' is 'g'.
'roll'+'in'+'g'='ROLLING'
(Other definitions for rolling that I've seen before include "Describes a range of rounded hills" , "Moving on wheels" , "Going round - a type of mill" , "being so drunk?" , "-- Stock; -- pin" .)