A dog's dinner perhaps for those who know their onions and what helps them grow (8)
I believe the answer is:
bonemeal
'for those who know their onions and what helps them grow' is the definition.
The definition suggests a singular noun which matches the answer.
'a dog's dinner' is the wordplay.
'a dog's' becomes 'bone' (I can't justify this - if you can you should give a lot more credence to this answer).
'dinner' becomes 'meal' (I've seen this in other clues).
'bone'+'meal'='BONEMEAL'
'perhaps' is the link.
Can you help me to learn more?
(Other definitions for bonemeal that I've seen before include "Organic fertiliser" , "Type of fertiliser" .)