Average quantity is most important (9)
I believe the answer is:
paramount
'most important' is the definition.
(I've seen this in another clue)
'average quantity' is the wordplay.
'average' becomes 'par' (average score in golf).
'quantity' becomes 'amount' (synonyms).
'par'+'amount'='PARAMOUNT'
'is' acts as a link.
(Other definitions for paramount that I've seen before include "First" , "Of the greatest importance" , "outstanding" , "Highest" , "Pre-eminent - movie maker" .)