Refuse to obey when fed up before end of day (4)
I believe the answer is:
defy
'refuse to obey' is the definition.
(I've seen this before)
'fed up before end of day' is the wordplay.
'fed up' becomes 'def' ('fed' going up in a down clue).
'before' is a charade indicator (letters next to each other).
'end of' indicates one should take the final letters.
The final letter of 'day' is 'y'.
'def'+'y'='DEFY'
'when' acts as a link.
(Other definitions for defy that I've seen before include "Go against (authority)" , "Stand against" , "Refuse to obey (person)" , "Openly resist or refuse to obey" , "Challenge or resist" .)