Quantity is changed to ten (3)
I believe the answer is:
six
'quantity' is the definition.
(six is a kind of quantity)
'is changed to ten' is the wordplay.
'changed' indicates anagramming the letters.
'to ten' becomes 'x' (roman numeral).
'is' is an anagram of 'si'.
'si'+'x'='SIX'
(Other definitions for six that I've seen before include "Airborne boundary hit" , "Half-dozen" , "VI" , "Half a dozen" , "Number of US moon landings" .)