Cat eating a rat's head? Speak with it! (6)
I believe the answer is:
larynx
'speak with it' is the definition.
(the larynx is the voicebox)
'cat eating a rat's head?' is the wordplay.
'cat' becomes 'lynx' (lynx is a kind of cat).
'eating' indicates putting letters inside.
'head?' indicates taking the first letters (I've seen 'heads of' mean this).
The initial letter of 'rats' is 'r'.
'a'+'r'='ar'
'lynx' enclosing 'ar' is 'LARYNX'.
(Other definitions for larynx that I've seen before include "The formal voice-box" , "Cavity containing vocal chords" , "Upper part of the windpipe" , "The voice box, part of throat" , "Human voice box" .)