Charged entities in bulbs not functioning (4)
I believe the answer is:
ions
'charged entities' is the definition.
'bulbs not functioning' is the wordplay.
'bulbs' becomes 'onions' (onion is a kind of bulb).
'not' suggests deleting specific letters.
'functioning' becomes 'on' (I've seen this in other clues).
'onions' with 'on' removed is 'IONS'.
'in' is the link.
(Other definitions for ions that I've seen before include "Electrically-charged particles" , "Electrically charged particles" , "N in radio comms." , "items with a charge" , "Charged atom or molecule" .)