Best frozen treat, no longer cold in the middle (6)
I believe the answer is:
choice
'best frozen treat' is the definition.
'choice' can be an answer for 'best' (both can mean the most excellent). I am not sure about the 'frozen treat' bit.
'no longer cold in the middle' is the wordplay.
'no longer' becomes 'choi' (I can't justify this - if you can you should believe this answer much more).
'cold' becomes 'iced' (similar in meaning).
'in the middle' says to take the centre.
The middle of 'iced' is 'ce'.
'choi'+'ce'='CHOICE'
Can you help me to learn more?
(Other definitions for choice that I've seen before include "Very good" , "Possibility" , "Act or power of selecting" , "Top quality" , "'Option, alternative (6)'" .)