Crossword Genius

Heard man thinks highly of fertilisers (8)

Ross

I believe the answer is:

nitrates

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'heard man thinks' is the definition.
Although both the answer and definition are plural nouns, I don't see how they can define each other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for nitrates that I've seen before include "Fertiliser salts found in train set" , "Compounds" .)

I've seen this clue in The Telegraph.
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