Jack wants something with milk in it — a shake (6)
I believe the answer is:
judder
'a shake' is the definition.
(I've seen this before)
'jack wants something with milk in it' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'wants' is a charade indicator (letters next to each other).
'something with milk in it' becomes 'udder' (eg a cow's udder).
'j'+'udder'='JUDDER'
(Other definitions for judder that I've seen before include "Shake or vibrate rapidly" , "Strong vibration" , "Vibrate violently" , "Shake/vibrate forcefully" .)