Tenth of a decade (4)
I believe the answer is:
year
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tenth of a decade</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 year that I've seen before include "See 19" , "Time taken by a planet to orbit the sun" , "Period of earth's orbit" , "One orbit round the sun" , "Student group" .)