Heard man thinks highly of fertilisers (8)
I believe the answer is:
nitrates
'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" .)