Behaving (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">behaving</span>' is the definition.<br/ ><span class="explanation-format__explanation">(acting is a kind of behaving)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for acting that I've seen before include "Taking a part - on a temporary basis" , "On stage; standing in for" , "Stand-in" , "Playing a part" , "Behaving in a certain way" .)