Transport setback leads to quarrel (4,2)
I believe the answer is:
bust up
'quarrel' is the definition.
(bust-up is a kind of quarrel)
'transport setback leads to' is the wordplay.
'transport' becomes 'bus' (bus is a kind of transport).
'setback' shows that the letters should be reversed in order.
'leads to' becomes 'put' (I can't justify this - if you can you should give a lot more credence to this answer).
'put' back-to-front is 'tup'.
'bus'+'tup'='BUST-UP'
Can you help me to learn more?
(Other definitions for bust up that I've seen before include "quarrel!" , "Bra-destroying feminist presumably wouldn't have this" , "disturbance" , "fight" .)