Certainty; something easy (5)
I believe the answer is:
cinch
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">certainty</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">something easy</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for cinch that I've seen before include "piece of cake" , "A walk in the park" , "In slang, a certainty or easy task" , "simple job" , "Something easily done" .)