Leg muscle a little lower? (4)
I believe the answer is:
calf
This clue is a double definition.
'leg muscle' is the first definition.
(I've seen this in another clue)
'a little lower?' is the second definition.
(little cow - 'low' can mean to make a mooing sound)
(Other definitions for calf that I've seen before include "Roo's mum (A. A. Milne)" , "Bit of leg" , "Fine leather - young whale" , "Animal that yields veal" , "Back of lower leg" .)