Strain loading contents of short chest (6)
I believe the answer is:
thorax
'chest' is the definition.
(I know that chest can be written as thorax)
'strain loading contents of short' is the wordplay.
'strain' becomes 'tax' (both can mean to overwork or demand a lot from).
'loading' indicates putting letters inside (inserted letters loaded into another word).
'contents of' indicates the central letters (the letters on the inside of the word).
The central letters of 'short' are 'hor'.
'tax' placed around 'hor' is 'THORAX'.
(Other definitions for thorax that I've seen before include "Chest" , "Between the neck and belly" , "Upper part of body" , "Middle section of an insect" , "Part of the body between neck and abdomen" .)