Juvenile not finishing cold food (8)
I believe the answer is:
childish
'juvenile' is the definition.
(I've seen this before)
'not finishing cold food' is the wordplay.
'not finishing' means to remove the last letter.
'cold' becomes 'chill' (chill can mean cold).
'food' becomes 'dish' (dish is a kind of food).
'chill' with its last letter taken off is 'chil'.
'chil'+'dish'='CHILDISH'
(Other definitions for childish that I've seen before include "Puerile" , "Silly, immature" , "Juvenile" .)