Childhood plumpness (5,3)
I believe the answer is:
puppy fat
'childhood plumpness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for puppy fat that I've seen before include "Childhood tubbiness, often lost by maturity" , "Early chubbiness" , "Youthful plumpness" , "Childhood plumpness" , "young blubber" .)