How often idiots appear in compound? (7)

I believe the answer is:

nitrate

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'compound?' is the definition.
(I know that nitrate is a type of chemical compound)

I can't explain the remainder of the clue.

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(Other definitions for nitrate that I've seen before include "Compound" , "following 28 [SILVER]" , "Tear tin up for fertiliser" , "Natural or synthetic fertiliser" , "salt" .)

I've seen this clue in The Sun.

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