Mum (6)
I believe the answer is:
silent
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mum</span>' is the definition.<br/ ><span class="explanation-format__explanation">(to keep mum can informally mean to stay silent)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for silent that I've seen before include "Strange and astonishing thing" , "Listen (anag.)" , "Unheard" , "Noiseless" , "Taciturn" .)