A rise in quantity (6)
I believe the answer is:
amount
'quantity' is the definition.
(I know that quantity can be written as amount)
'a rise' is the wordplay.
'rise' becomes 'mount' (both can mean to increase).
'a'+'mount'='AMOUNT'
'in' acts as a link.
(Other definitions for amount that I've seen before include "Supply" , "Total or extent" , "Sum of money" , "value" , "Add up or be equivalent to" .)