From dark heap picked up a sort of acid (6)

I believe the answer is:

nitric

'a sort of acid' is the definition.
(nitric acid)

I don't understand the rest of the clue.

(Other definitions for nitric that I've seen before include "Citrin (anag.)" , "Very corrosive acid type" , "kind of acid" , "Of or containing nitrogen" , "Colourless highly corrosive acid" .)

I've seen this clue in The Times.

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