One of twelve, impressive (6)
I believe the answer is:
august
I believe this clue is a double definition.
'one of twelve' is the first definition.
(one of the twelve months)
'impressive' is the second definition.
(I've seen this before)
(Other definitions for august that I've seen before include "Sublime" , "For Edna O'Brien, it was a 27 across month" , "Period in 1 down" , "Gregorian calendar month" , "Impressive, majestic" .)