Period when discharge comes up (4)
I believe the answer is:
time
'period' is the definition.
(time is a kind of period)
'discharge comes up' is the wordplay.
'discharge' becomes 'emit' (emitting is a kind of discharging).
'comes up' says the letters should be written backwards.
'emit' back-to-front is 'TIME'.
'when' is the link.
(Other definitions for time that I've seen before include "The father of Truth, presumably" , "It's done in prison" , "Bird" , ""The enemy"" , "Great healer, so they say" .)