Romaine lettuce (3)
I believe the answer is:
cos
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">romaine lettuce</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cos that I've seen before include "Variety of lettuce" , "Lettuce with long crisp leaves" , "often associated with sin" , "Salad ingredient" , "Greek island and type of lettuce" .)