Cat crossing a river has a bit of neck (6)
I believe the answer is:
larynx
'a bit of neck' is the definition.
('larynx' is part of 'neck')
'cat crossing a river' is the wordplay.
'cat' becomes 'lynx' (lynx is a kind of cat).
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'a'+'r'='ar'
'lynx' enclosing 'ar' is 'LARYNX'.
'has' acts as a link.
(Other definitions for larynx that I've seen before include "Creator of speech" , "Organ containing the vocal cords" , "Location of vocal cords" , "Cavity containing vocal chords" , "part of pipe" .)