It's transported to the poles in metal boxes (4)
I believe the answer is:
tins
'metal boxes' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps there's an association between them I don't understand?
'it's transported to the poles' is the wordplay.
'transported' indicates anagramming the letters.
'to the poles' becomes 'ns' (North and South poles).
'it' with letters rearranged gives 'ti'.
'ti'+'ns'='TINS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for tins that I've seen before include "food containers" , "Cans" .)