Reserve put on the best dessert (3,5)
I believe the answer is:
ice cream
'dessert' is the definition.
(ice cream is a kind of dessert)
'reserve put on the best' is the wordplay.
'reserve' becomes 'ice' (I've seen this before).
'put on' says to put letters next to each other.
'the best' becomes 'cream' (I've seen this before).
'ice'+'cream'='ICE-CREAM'
(Other definitions for ice cream that I've seen before include "Creamy frozen confection, best in summer" , "Cold snack" , "Sweet frozen foodstuff" , "Cool confection, sweet frozen dessert" , "chilling creation" .)