Mallet finally breaking bones in fish (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(sturgeon is a kind of fish)
'mallet finally breaking bones' is the wordplay.
'finally' suggests the final letters.
'breaking' is an insertion indicator.
'bones' becomes 'surgeon' (I can't justify this - if you can you should give a lot more credence to this answer).
The final letter of 'mallet' is 't'.
't' inserted into 'surgeon' is 'STURGEON'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for sturgeon that I've seen before include "Fish that yields caviare" , "The fish that yields caviar" , "Fish yielding caviar and isinglass" , "Main resident" , "swimmer" .)