One won't want milk, say, being kept in vehicle (5)
I believe the answer is:
vegan
'one won't want milk' is the definition.
'say being kept in vehicle' is the wordplay.
'say' becomes 'eg'.
'being kept in' means one lot of letters goes inside another.
'vehicle' becomes 'van' (van is a kind of vehicle).
'eg' put inside 'van' is 'VEGAN'.
(Other definitions for vegan that I've seen before include "One shunning animal products" , "Avoiding animal products" , "Selective consumer" , "One refusing animal products" , "An extreme vegetarian" .)