Nine failed in test occurring every third year (9)
I believe the answer is:
triennial
'occurring every third year' is the definition.
(I've seen this in another clue)
'nine failed in test' is the wordplay.
'failed' is an anagram indicator.
'in' means one lot of letters goes inside another.
'test' becomes 'trial' (synonyms).
'nine' with letters rearranged gives 'ienn'.
'ienn' inserted inside 'trial' is 'TRIENNIAL'.
(Other definitions for triennial that I've seen before include "Regularly occurring" , "lasting a few years" , "Occurring every three years" , "Recurring every three years" .)