Mother superior (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mother superior</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for abbess that I've seen before include "Religious leader" , "woman trying to be religious?" , "Superior of convent" , "Senior ecclesiastic" , "Head nun" .)