Mum is reversing fast (6)
I believe the answer is:
silent
'mum' is the definition.
(to keep mum can informally mean to stay silent)
'is reversing fast' is the wordplay.
'reversing' says the letters should be written in reverse.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'is' written backwards gives 'si'.
'si'+'lent'='SILENT'
(Other definitions for silent that I've seen before include "Utterly quiet" , "Making no sound" , "unable to speak?" , "Without sound" , "Anagram of 7 Down" .)