Youngster devouring hot fish (4)
I believe the answer is:
chub
'fish' is the definition.
(chub is a kind of fish)
'youngster devouring hot' is the wordplay.
'youngster' becomes 'cub' (young bear).
'devouring' means one lot of letters goes inside another.
'hot' becomes 'h' (common abbreviation eg on taps).
'cub' going around 'h' is 'CHUB'.
(Other definitions for chub that I've seen before include "European freshwater fish" , "Thick-bodied European fish" , "Fat freshwater fish" , "Plump fish" , "River fish; rhymes with public house" .)