Cat eats a rat's head and part of respiratory tract (6)
I believe the answer is:
larynx
'part of respiratory tract' is the definition.
('larynx' is part of 'respiratory tract')
'cat eats a rat's head' is the wordplay.
'cat' becomes 'lynx' (lynx is a kind of cat).
'eats' indicates putting letters inside.
'head' says to take the initial letters.
The first letter of 'rats' is 'r'.
'a'+'r'='ar'
'lynx' going around 'ar' is 'LARYNX'.
'and' is the link.
(Other definitions for larynx that I've seen before include "Source of squeaks" , "Where growls come from" , "Upper part of the windpipe" , "it's essential to speak!" , "part of neck" .)