Happening every second year (8)
I believe the answer is:
biennial
'happening every second year' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for biennial that I've seen before include "Plant with a two-year life cycle" , "Taking place every other year" , "Two-yearly" , "Plant living two years" , "Occurring every second year" .)