Quieten a beast with noisy innards (6)
I believe the answer is:
muffle
'quieten' is the definition.
(muffling is a kind of quietening)
'a beast with noisy innards' is the wordplay.
I cannot really see how this works, but
'noisy' could be 'ff' and 'ff' is found within the answer.
'beast' could be 'mule' (I have seen 'Stubborn beast' mean 'mule' so perhaps 'beast' could also mean 'mule') and 'mule' is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for muffle that I've seen before include "Deaden, a sound or noise" , "Keep warm" , "silence" , "Deaden(er of) sound" , "Envelop" .)