Lettuce (3)

I believe the answer is:
cos
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lettuce</span>' is the definition.<br/ ><span class="explanation-format__explanation">(cos lettuce is a kind of lettuce)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cos that I've seen before include "vegetable?" , "holiday destination" , "Chief of Staff" , "Kind of lettuce in microcosm" , "something of a salad" .)