Missing some exercise for boxers (8)
I believe the answer is:
skipping
This might be a double definition.
'missing some' is the first definition.
'skipping' can be an answer for 'missing' (skipping is a kind of missing). I'm not sure about the 'some' bit.
'exercise for boxers' is the second definition.
'skipping' can be an answer for 'exercise' (skipping is a type of physical exercise). I'm not sure about the 'for boxers' bit.
Can you help me to learn more?
(Other definitions for skipping that I've seen before include "Jumping lightly or bypassing" , "amusement" , "Jumping with a rope" , "Cutting" , "Missing - physical exercise" .)