Costs nothing to let them go (4)

I believe the answer is:

free

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'go' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for free that I've seen before include "Liberal" , "Available - for love" , "Not occupied; release" , "Unrestricted" , "Deliver" .)

I've seen this clue in The Irish Times.

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