Shakes? Not Juliet's source of milk (6)
I believe the answer is:
udders
'source of milk' is the definition.
'shakes? not juliet's' is the wordplay.
'shakes?' becomes 'judders' ('judder' can be a synonym of 'shake').
'not' indicates named letters should be taken away.
'juliet' becomes 'j' (phonetic alphabet: alpha, bravo, charlie etc.).
'judders' with 'j' taken out is 'UDDERS'.
(Other definitions for udders that I've seen before include "Teats of a cow" , "de rest" , "Containers for milk, naturally!" , "Cows' teats" , "Bovine mammary glands" .)