Two-thirds of a dozen (5)
I believe the answer is:
eight
'two-thirds of a dozen' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eight that I've seen before include "Number etched by a skater?" , "power of two" , "Hit, e.g. (anag.)" , "Have one over the . . . . . to be slightly drunk" , "Pints in gallon" .)