One of those carrying weight perhaps has a lot on his shoulders (6)
I believe the answer is:
lifter
'those carrying weight perhaps has a lot on his shoulders' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'li' (I've seen this in other clues) and 'li' is present in the answer.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for lifter that I've seen before include "One putting up" , "One who raises things like weights" .)