Once in a while (3,3,4)
I believe the answer is:
now and then
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">once in a while</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for now and then that I've seen before include "Occasionally" , "Sometimes" , "At times" , "making a historical contrast" .)