Honest about the quantity (3)
I believe the answer is:
one
'the quantity' is the definition.
(one is a kind of quantity)
'honest about' is the wordplay.
'about' says the answer is hidden in the clue.
'ONE' can be found hidden inside 'honest'.
(Other definitions for one that I've seen before include "Sole; number" , "Unity" , "Lowest cardinal number" , "A certain; single person" , "Half of two" .)