Quantity is under par, but still supreme (9)
I believe the answer is:
paramount
'still supreme' is the definition.
'paramount' can be an answer for 'supreme' (I've seen this in another clue). I am unsure of the 'still' bit.
'quantity is under par' is the wordplay.
'quantity' becomes 'amount' (synonyms).
'is under' is a charade indicator (letters next to each other).
'amount' put after 'par' is 'PARAMOUNT'.
'but' is the link.
Can you help me to learn more?
(Other definitions for paramount that I've seen before include "Pre-eminent - movie maker" , "Superior to all others" , "Predominant" , "Supreme - Hollywood studio" , "Of overriding importance" .)