A well-known flower fertiliser (6)
I believe the answer is:
pollen
'fertiliser' is the definition.
(pollen fertilises plants)
'a well-known flower' is the wordplay.
I cannot quite understand how this works, but
'flower' could be 'po' (Italian river) and 'po' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for pollen that I've seen before include "Powdery substance given off by flowers" , "Powdery stuff produced by plants" , "Fine powder given off by plants" , "Fertilising dust from flower" , "Fine powder from certain plants" .)