One month (6)
I believe the answer is:
august
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one month</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that August is a type of Gregorian calendar month)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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]" .)