Butchery that he set in motion, should we forget and forgive? (4,3,7)
I believe the answer is:
bury the hatchet
'motion should we forget and forgive?' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'butchery that he set' is the wordplay.
'set' indicates an anagram (instruction to set the letters).
'butchery'+'that'+'he'='butcherythathe'
'butcherythathe' with letters rearranged gives 'BURY THE HATCHET'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for bury the hatchet that I've seen before include "end feud - with 18d" , "Become reconciled" , "having decided against axing?" , "Settle quarrel" , "sinking of cutter" .)