A little lower - a little lower than the knee (4)
I believe the answer is:
calf
This clue is a triple definition!
'a little lower' is the first definition.
(little cow - 'low' can mean to make a mooing sound)
'a little lower' is the second definition.
(little cow - 'low' can mean to make a mooing sound)
'than' is a link.
'the knee' is the third definition.
(thesaurus)
(Other definitions for calf that I've seen before include "Young elephant" , "Young bovine animal" , "Is it above the ankle" , "Baby elephant or whale" , "Young cow/bull" .)