Insisted on a fresh dessert (8)
I believe the answer is:
asserted
'insisted' is the definition.
(asserting is a kind of insisting)
'on a fresh dessert' is the wordplay.
'on' says to put letters next to each other.
'fresh' indicates anagramming the letters (a fresh re-ordering of the letters).
'dessert' is an anagram of 'sserted'.
'a' next to 'sserted' is 'ASSERTED'.
(Other definitions for asserted that I've seen before include "Confidently declared to be so" , "Stated firmly, declared." , "Made firm statement about a dessert" , "Put" .)