Free of charge (6)

'free of charge' is the definition.
(synonyms)
This is all the clue.
(Other definitions for gratis that I've seen before include "Without charge" , "Free, at no cost" , "Costing nothing, free" , "Free, costs nothing" , "not charged" .)
(Other definitions for unload that I've seen before include "Remove (a gun's ammo)" , "Get rid" , "Remove goods from (a vehicle or ship)" , "Discharge cargo" , "Remove cargo from ship or goods from lorry" .)
'free of charge' is the definition.
(I've seen this before)
'of charge' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is found in the answer.
'charge' could be 'rate' (rate is a kind of charge) and 'rate' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for exonerate that I've seen before include "let off the hook" , "Officially absolve from blame" , "Release" , "Free from obligation, blame" , "Pronounce not guilty of a crime" .)
'free of charge' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for buckshee that I've seen before include "Slangily free of charge" , "Free of charge, gratis" , "Free, gratis and for nothing" .)
(Other definitions for unburden that I've seen before include "Ease the load" , "Relieve of a load" , "Remove load" .)