Load of oxygen: setter coming round (4)
I believe the answer is:
onus
'load' is the definition.
(I know that load can be written as onus)
'oxygen setter coming round' is the wordplay.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'setter' becomes 'sun' (the sun sets every day).
'coming round' says the letters should be written backwards.
'sun' written backwards gives 'nus'.
'o'+'nus'='ONUS'
'of' acts as a link.
(Other definitions for onus that I've seen before include "Responsibilty" , "albatross?" , "'Responsibility, task or burden (4)'" , "Obligation - charge" , "Burden, duty" .)