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