Mum is coming over fast (6)
I believe the answer is:
silent
'mum' is the definition.
(to keep mum can informally mean to stay silent)
'is coming over fast' is the wordplay.
'coming over' is a reversal indicator.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'is' in reverse letter order is 'si'.
'si'+'lent'='SILENT'
(Other definitions for silent that I've seen before include "Noiseless" , "Unheard" , "Inaudible" , "still" , "Lacking sound" .)