Heavy mallets used for crushing insects (7)
I believe the answer is:
beetles
'insects' is the definition.
(beetle is a kind of insect)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for beetles that I've seen before include "Lets bee turn to insects" , "Four-winged insects" , "Bet eels could turn into insects" .)