A dessert not usually insisted upon (8)
I believe the answer is:
asserted
'insisted upon' is the definition.
I can't judge whether this defines the answer.
'a dessert not usually' is the wordplay.
'not usually' is an anagram indicator (an unusual ordering of the letters).
'dessert' with letters rearranged gives 'sserted'.
'a'+'sserted'='ASSERTED'
Can you help me to learn more?
(Other definitions for asserted that I've seen before include "Made firm statement about a dessert" , "Put" , "Confidently declared to be so" , "Stated firmly, declared." .)