Annum (4)
I believe the answer is:
year
'annum' is the definition.
(I know that annum can be written as year)
This is the entire clue.
(Other definitions for year that I've seen before include "Two semesters" , "Period of 5 across" , "One intake of undergrads" , "Time taken by a planet to orbit the sun" , "Period of revolution" .)