Crossword Genius

Make free of charge (6)

Ross

I believe the answer is:

unload

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'make' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how they can define each other.

'free of charge' is the wordplay.
'free of' becomes 'un' (I can't justify this - if you can you should believe this answer much more).
'charge' becomes 'load' (synonyms).
'un'+'load'='UNLOAD'

Can you help me to learn more?

(Other definitions for unload that I've seen before include "Remove cargo from ship or charge from gun" , "Take cargo off (a ship)" , "Discharge cargo" , "Get rid" , "Remove (a gun's ammo)" .)

I've seen this clue in the Evening Standard.
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