A dozen (6)
I believe the answer is:
twelve
'a dozen' is the definition.
('twelve' can be a synonym of 'dozen')
This is all the clue.
(Other definitions for twelve that I've seen before include "Lunchtime" , "Number of centuries needed for MCC" , "Apostles and members of the jury" , "Midnight or noon" , "One under a baker's dozen" .)