Started smuggling sticky black stuff (3)

I believe the answer is:

tar

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

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

'started smuggling' is the wordplay.
'TAR' is hidden in amongst the letters.
But, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for tar that I've seen before include "Sailor (colloq.)" , "Seaman; pitch" , "It paves the way" , "Crewman" , "Smoker's black deposit" .)

I've seen this clue in The Mirror.

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