Quiet person eating second dessert (6)
I believe the answer is:
mousse
'dessert' is the definition.
(mousse is a kind of dessert)
'quiet person eating second' is the wordplay.
'quiet person' becomes 'mouse' (I've seen this before).
'eating' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'mouse' placed around 's' is 'MOUSSE'.
(Other definitions for mousse that I've seen before include "Creamy substance used for styling hair" , "Whipped cream and egg dish" , "Foamy dish" , "Gel" , "Creamy dessert; hair dressing" .)