Mum is receding fast (6)
I believe the answer is:
silent
'mum' is the definition.
('silent' can be a synonym of 'mum')
'is receding fast' is the wordplay.
'receding' says the letters should be written in reverse.
'fast' becomes 'lent' (I've seen this before).
'is' in reverse letter order is 'si'.
'si'+'lent'='SILENT'
(Other definitions for silent that I've seen before include "Unspeaking" , "Like William Prince of Orange" , "Listen in order to be quiet" , "Wordless" , "Dumb" .)