Working copper brought in librarian, a fierce young thing (4,3)
I believe the answer is:
lion cub
'a fierce young thing' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'working copper brought in librarian' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'brought in' is an insertion indicator.
'librarian' becomes 'lib' (abbreviation for librarian).
'on'+'cu'='oncu'
'oncu' going inside 'lib' is 'LION-CUB'.
Can you help me to learn more?
(Other definitions for lion cub that I've seen before include "Baby feline" , "Young pride member" , "Young mauler" , "Elsa's offspring possibly (Born Free)" , "Young big cat" .)