Enormous cut in arm (8)
I believe the answer is:
whacking
'enormous' is the definition.
(I've seen this before)
'cut in arm' is the wordplay.
I cannot quite see how this works, but
'cut' could be 'hack' (hack can mean to cut or chop) and 'hack' is found within the answer.
'in' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for whacking that I've seen before include "Very big" , "hitting" , "enormous" .)