Reserve the best dessert? (3,5)
I believe the answer is:
ice cream
'dessert?' is the definition.
(ice cream is a kind of dessert)
'reserve the best' is the wordplay.
'reserve' becomes 'ice' (I've seen this before).
'the best' becomes 'cream' (I've seen this before).
'ice'+'cream'='ICE-CREAM'
(Other definitions for ice cream that I've seen before include "Sweet frozen food" , "Cold confection" , "Frozen food made with milk fat" , "Creamy frozen confection, best in summer" , "Hot day treat" .)