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' (busing is a kind of transporting).
'setback' shows that the letters should be reversed in order.
'leads to' becomes 'put' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'put' written backwards gives '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" .)