Jack has to sing out, looking amazed (3)

I believe the answer is:

tar

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'jack' is the definition.
(both are informal terms for sailor)

I don't understand the rest of the clue.

(Other definitions for tar that I've seen before include "Thick flammable liquid" , "Black coat" , "Cigarette residue - old word for sailor" , "Jack ..., British sailor" , "Product of coal" .)

I've seen this clue in the Evening Standard.

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