Barge - partly unloaded? (7)

I believe the answer is:

lighter

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'barge' is the definition.
(I know that lighter is a type of barge)

I can't explain the rest of the clue.

(Other definitions for lighter that I've seen before include "Vessel" , "Barge (more likely to float?)" , "Boat for heavy loads and fire igniter" , "relieved of some cargo?" , "Fire-raiser" .)

I've seen this clue in The Irish News.

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