Honest about quantity (3)
I believe the answer is:
one
'quantity' is the definition.
(one is a kind of quantity)
'honest about' is the wordplay.
'about' indicates the answer is hidden within the clue (the clue goes around or about the answer).
'ONE' can be found hidden inside 'honest'.
(Other definitions for one that I've seen before include "Smallest whole number" , "Figure" , "Hour after midnight" , "Single, sole" , "doesn't count for much" .)