Mum let sin get out of control (6)
I believe the answer is:
silent
'mum' is the definition.
(to keep mum can informally mean to stay silent)
'let sin get out of control' is the wordplay.
'get out of control' indicates an anagram.
'let'+'sin'='letsin'
'letsin' with letters rearranged gives 'SILENT'.
(Other definitions for silent that I've seen before include "Free from noise" , "Making no noise" , "Making no sound" , "Unheard" , "Listen (anag.)" .)