Give out when time's up (4)
I believe the answer is:
emit
'give out' is the definition.
(I know that give out can be written as emit)
'time's up' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'time' reversed gives 'EMIT'.
'when' acts as a link.
(Other definitions for emit that I've seen before include "Produce and discharge" , "Send out in time" , "Release" , "Give out (eg light)" , "Make audible sounds" .)