Last month (6)
'last month' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ultimo that I've seen before include "Previous month" , "In full, last month in business correspondence" , "Indicating last month, usually abbreviated" , "In commerce, of last month" , "(Dated) of last month" .)
'last month' is the definition.
(last month of the year)
This is the entire clue.
(Other definitions for december that I've seen before include "Winter month" , "12th month" , "Original tenth month" , "Tenth month to the old Romans" , "Last month" .)