They're not free of charge (4)
I believe the answer is:
ions
'they're not free of charge' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ions that I've seen before include "items with a charge" , "Charged atom or molecule" , "N in radio comms." , "Electrically charged particles" , "Electrically-charged particles" .)