Erica messily eats large dessert (6)
I believe the answer is:
eclair
'dessert' is the definition.
(I know that eclair is a type of dessert)
'erica messily eats large' is the wordplay.
'messily' indicates an anagram.
'eats' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'erica' with letters rearranged gives 'ecair'.
'ecair' going around 'l' is 'ECLAIR'.
(Other definitions for eclair that I've seen before include "Long, individual cake" , "Long chocolate cake for Claire" , "I clear off with thin chocolate cake" , "Bakery item" , "comedian Jenny?" .)