They are used for holding coal fires (5)
I believe the answer is:
sacks
'fires' is the definition.
('sack' can be a synonym of 'fire')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sacks that I've seen before include "Bags; wines" , "Discharges" , "Raids; boots out" , "rifles" , "Large strong bags" .)