A little lower - than the knee (4)
I believe the answer is:
calf
This clue is a double definition.
'a little lower' is the first definition.
(little cow - 'low' can mean to make a mooing sound)
'the knee' is the second definition.
(thesaurus)
'than' is the link.
(Other definitions for calf that I've seen before include "A little lower" , "Back of lower leg" , "Young cow/bull" , "Part of the leg - baby farm animal" , "Baby elephant or whale" .)