Theatrical knife (7)
I believe the answer is:
scalpel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">theatrical knife</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 scalpel that I've seen before include "sharp knife" , "Surgical instrument" , "Medical knife" , "One making cuts in hospital" , "Sharp blade" .)