Having surgery (5,3,5)
I believe the answer is:
under the knife
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">having surgery</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for under the knife that I've seen before include "having an operation" , "Experiencing surgery" , "Being operated upon" .)