It means trouble when a vehicle gets put in reverse (4-2)
I believe the answer is:
bust-up
'it means trouble' is the definition.
The definition suggests a singular noun which matches the answer.
'a vehicle gets put in reverse' is the wordplay.
'a vehicle' becomes 'bus' (bus is a kind of vehicle).
'gets' means one lot of letters go next to another.
'in reverse' says the letters should be written backwards.
'put' in reverse letter order is 'tup'.
'bus'+'tup'='BUST-UP'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for bust-up that I've seen before include "Quarrel, brawl" , "Quarrel, possibly ending a friendship" , "Argument finishing a relationship" , "Serious quarrel/fight" , "Serious altercation" .)