Diary capacity (4)
I believe the answer is:
year
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">diary capacity</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for year that I've seen before include "Time taken by a planet to orbit the sun" , "One orbit round the sun" , "Complete cycle of seasons" , "Four seasons" , "Group of students" .)