Mum ordered tinsel (6)
I believe the answer is:
silent
'mum' is the definition.
(to keep mum can informally mean to stay silent)
'ordered tinsel' is the wordplay.
'ordered' indicates an anagram.
'tinsel' is an anagram of 'SILENT'.
(Other definitions for silent that I've seen before include "Making no noise" , "Anagram of 7 Down" , "Free from noise" , "Wordless" , "Lacking sound" .)