Load nothing before sun-up (4)
I believe the answer is:
onus
'load' is the definition.
('onus' can be a synonym of 'load')
'nothing before sun-up' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'before' is a charade indicator (letters next to each other).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'sun' in reverse letter order is 'nus'.
'o'+'nus'='ONUS'
(Other definitions for onus that I've seen before include "'Burden, of proof (4)'" , "'Burden, load (4)'" , "'Responsibility, task or burden (4)'" , "Liability" , "Burden, duty" .)