Throw out — time's up (4)
I believe the answer is:
emit
'throw out' is the definition.
(I've seen this before)
'time's up' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'time' in reverse letter order is 'EMIT'.
(Other definitions for emit that I've seen before include "Issue" , "Throw out" , "Let out" , "Give out (eg light)" , "Come out!" .)