Crossword Genius

They burn sacks (5)

Ross

I believe the answer is:

fires

I'm a little stuck... Click here to teach me more about this clue!

'sacks' is the definition.
('fire' can be a synonym of 'sack')

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

Can you help me to learn more?

(Other definitions for fires that I've seen before include "Sacks; conflagrations" , "Cashiers" .)

I've seen this clue in The Irish News.
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