Crossword Genius

What drug smuggler did heartlessly is foul (6)

Ross

I believe the answer is:

rancid

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

I don't understand the remainder of the clue.

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(Other definitions for rancid that I've seen before include "(Of fatty foods) stale" , "(Of butter) stale" , "Stinking" , "Sour, rotting" , "Rank in smell and taste" .)

I've seen this clue in The Independent.
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