Occurring once every year (6)
I believe the answer is:
annual
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">occurring once every year</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for annual that I've seen before include "yearly event" , "Type of year book" , "Coming once a year" , "(Of plant) living for a year or less" , "Occurring once every year" .)