Period of ten years (6)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">period of ten years</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for decade that I've seen before include "Ten consecutive years" , "Ten-year period" , "number of years" , "The noughties maybe" , "Time period" .)