The cat turns on it without a sound (5)
I believe the answer is:
tacit
'without a sound' is the definition.
(tacit can mean implied or unspoken)
'the cat turns on it' is the wordplay.
'turns' says the letters should be written in reverse.
'on' is a charade indicator (letters next to each other).
'cat' reversed gives 'tac'.
'tac'+'it'='TACIT'
(Other definitions for tacit that I've seen before include "Silent but implied" , "Implied without being put into words" , "Unrecorded" , "without speaking" , "Aircraft engine casing" .)