Butcher lark during rest break (9)
I believe the answer is:
slaughter
'butcher' is the definition.
(I know that butcher can be written as slaughter)
'lark during rest break' is the wordplay.
'lark' becomes 'laugh' (I can't justify this - if you can you should give a lot more credence to this answer).
'during' means one lot of letters goes inside another.
'break' indicates anagramming the letters.
'rest' is an anagram of 'ster'.
'laugh' put into 'ster' is 'SLAUGHTER'.
Can you help me to learn more?
(Other definitions for slaughter that I've seen before include "Killing (of farm animals)" , "Butchery" , "Butcher" , "Mass killing or carnage" , "Widespread killing" .)