Month in advance (5)
I believe the answer is:
march
This clue is a double definition.
'month' is the first definition.
(I know that March is a type of Gregorian calendar month)
'advance' is the second definition.
(I know that march is a type of advance)
'in' is the link.
(Other definitions for march that I've seen before include "make progress" , "Month, piece of music" , "Military walking style" , "The month for military movement" , "Spring" .)