Mail department is taking on toxic stuff (6)

I believe the answer is:

poison

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'toxic stuff' is the definition.
(I've seen this in another clue)

'mail department is taking on' is the wordplay.
'mail department' becomes 'pois' (I can't explain this - if you can you should believe this answer much more).
'is taking' means one lot of letters go next to another.
'pois'+'on'='POISON'

Can you help me to learn more?

(Other definitions for poison that I've seen before include "Eg, toxin" , "Venom, toxin" , "Toxic substance" , "Eg, strychnine" , "Damaging stuff - drink?" .)

I've seen this clue in the Sydney Morning Herald.

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