Each year having a written test (5)
I believe the answer is:
paper
'written test' is the definition.
(exam paper)
'each year having a' is the wordplay.
'each year' becomes 'PA' (abbreviation for per annum).
'having' means one lot of letters go next to another.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'pa'+'per'='PAPER'
(Other definitions for paper that I've seen before include "Short essay" , "Set of examination questions" , "Telegraph, for example" , "Literary essay" , "Sort of clip" .)