Quantity of honesty (3)
I believe the answer is:
one
'quantity' is the definition.
(I know that one is a type of number)
'of honesty' is the wordplay.
'of' says the answer is hidden in the clue.
'ONE' is hidden in the letters of 'honesty'.
(Other definitions for one that I've seen before include "1" , "Lowest cardinal number" , "Ace" , "Sole; identical" , "A certain; single person" .)