Abnormal sac
I believe the answer is:
cyst
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">abnormal sac</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cyst that I've seen before include "pouch" , "Abnormal closed sac in a body" , "Bag of fluid" , "Thin-walled cavity containing liquid" , "Small bladder-like sac" .)