A little software distributed round vehicle, one proverbially unstable (5,4)
I believe the answer is:
apple cart
'a little software distributed' is the definition.
I know nothing about this answer so I can't judge whether this works.
'round vehicle one proverbially unstable' is the wordplay.
'round' indicates putting letters inside.
'vehicle' becomes 'car' (car is a kind of vehicle).
'one proverbially unstable' becomes 'applet' (I am not sure about this - if you are sure you should believe this answer much more).
'car' inserted within 'applet' is 'APPLE-CART'.
Can you help me to learn more?
(Other definitions for apple cart that I've seen before include "It gets upset" , "Upsetting subject" , "It's always being upset" , "you might upset it" , "this upset could throw plans into confusion" .)