A chap on board upset one shouldering great weight (5)
I believe the answer is:
atlas
'great weight' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'a chap on board upset one shouldering' is the wordplay.
'chap on board' becomes 'salt' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'upset' says the letters should be written backwards.
'one shouldering' means one lot of letters go next to another (some letters carry or shoulder others).
'salt' in reverse letter order is 'tlas'.
'a'+'tlas'='ATLAS'
Can you help me to learn more?
(Other definitions for atlas that I've seen before include "Size of paper once" , "N. African mountains; a Titan" , "In Greek myth, a Titan made to support the heavens" , "Set of charts" , "Book of maps - fifteenth satellite of Jupiter" .)