Discharge, in more ways than one (3,3)

I believe the answer is:

let off

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'discharge' is the definition.
(I've seen this before)

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

(Other definitions for let off that I've seen before include "Excuse from doing, or refrain from punishing" , "chance to avoid trouble" , "Release by exploding - pardon" , "Allow to escape punishment" , "Discharge (a firework)" .)

I've seen this clue in The Independent.

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