Highest quality fine grass turned pale (3-6)
I believe the answer is:
top-flight
'highest' is the definition.
(I've seen this before)
'fine grass turned pale' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'grass' becomes 'pot' (cannabis).
'turned' shows that the letters should be reversed in order.
'pale' becomes 'light' (similar in meaning).
'f'+'pot'='fpot'
'fpot' written backwards gives 'topf'.
'topf'+'light'='TOP-FLIGHT'
'quality' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for top-flight that I've seen before include "Superior" , "Highest level" , "Excellent -- best possible" , "Of the highest class" .)