Young bear catches last of fresh fish (4)
I believe the answer is:
chub
'fish' is the definition.
(chub is a kind of fish)
'young bear catches last of fresh' is the wordplay.
'young bear' becomes 'cub' (I've seen this in another clue).
'catches' indicates putting letters inside.
'last of' indicates one should take the final letters.
The last letter of 'fresh' is 'h'.
'cub' going around 'h' is 'CHUB'.
(Other definitions for chub that I've seen before include "Fat relative of the carp" , "River fish; rhymes with public house" , "Thick-bodied river fish" , "Plump fish" , "European freshwater fish" .)