Blubber one expects with youngster? (5,3)
I believe the answer is:
puppy fat
'blubber' is the definition.
(I have seen 'young blubber' mean 'puppy fat' so perhaps 'blubber' could also mean 'puppy fat')
'one expects with youngster?' is the wordplay.
I cannot quite see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
'youngster?' could be 'pup' (I've seen this before) and 'pup' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for puppy fat that I've seen before include "Adipose tissue disappearing in adolescence" , "Excess weight during youth" , "young blubber" , "Early chubbiness" , "Childhood plumpness" .)