Broken transport systems' core (4)
I believe the answer is:
bust
'broken' is the definition.
(bust can mean broken)
'transport systems' core' is the wordplay.
'transport' becomes 'bus' (bus is a kind of transport).
'core' indicates the central letters.
The central letter of 'systems' is 't'.
'bus'+'t'='BUST'
(Other definitions for bust that I've seen before include "economic collapse" , "Sculpture of head and chest" , "Broken - bankrupt - sculpture" , "Smash and grab" , "item of statuary" .)