Lets in squiffy mum (6)
I believe the answer is:
silent
'mum' is the definition.
(to keep mum can informally mean to stay silent)
'lets in squiffy' is the wordplay.
'squiffy' is an anagram indicator.
'lets'+'in'='letsin'
'letsin' with letters rearranged gives 'SILENT'.
(Other definitions for silent that I've seen before include "Unheard" , "Without sound" , "Strange and astonishing thing" , "Anagram of 7 Down" , "Like William Prince of Orange" .)