Time to come up for discharge (4)
I believe the answer is:
emit
'for discharge' is the definition.
(emitting is a kind of discharging)
'time to come up' is the wordplay.
'to come up' is a reversal indicator.
'time' written backwards gives 'EMIT'.
(Other definitions for emit that I've seen before include "Give off or discharge" , "Give out (eg light)" , "Give off, gas say" , "Give out or pour forth" , "Issue" .)