Only half of them wait for discharge (4)
I believe the answer is:
emit
'for discharge' is the definition.
(emitting is a kind of discharging)
'only half of them wait' is the wordplay.
'only half of' suggests halving the letters.
'wait' becomes 'it' (I can't explain this - if you can you should believe this answer much more).
'them' with half the letters taken is 'em'.
'em'+'it'='EMIT'
Can you help me to learn more?
(Other definitions for emit that I've seen before include "Release" , "Send forth" , "Small turtle" , "Throw out" , "Give out or pour forth" .)