Hard men punching to weaken part of the torso (6)
I believe the answer is:
thorax
'part of the torso' is the definition.
(I know this)
'hard men punching to weaken' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'men' becomes 'or' (military abbreviation for Other Ranks).
'punching' indicates putting letters inside (some letters punch a hole in others).
'to weaken' becomes 'tax' (tax can mean to exhaust or demand too much from).
'h'+'or'='hor'
'hor' going inside 'tax' is 'THORAX'.
(Other definitions for thorax that I've seen before include "Chest" , "Green light" , "Between the neck and belly" , "Insect's upper body" , "Upper part of body" .)