One month (6)
I believe the answer is:
august
'one month' is the definition.
(I know that August is a type of Gregorian calendar month)
This is the entire clue.
(Other definitions for august that I've seen before include ". . . . . . is a Wicked Month (Edna O'Brien)" , "For Edna O'Brien, it was a 27 across month" , "which is supposedly hot" , "Dignified - month" , "there's not much of it left now [27 AUGUST]" .)