Spell 'Discharge' backwards (4)
I believe the answer is:
time
'spell' is the definition.
(thesaurus)
'discharge backwards' is the wordplay.
'discharge' becomes 'emit' (emitting is a kind of discharging).
'backwards' says the letters should be written backwards.
'emit' reversed gives 'TIME'.
(Other definitions for time that I've seen before include "The father of Truth, presumably" , "Speech to close pub" , "See 1 and 21" , "It's done in prison" , "Indefinite period" .)