Back section of the carriage (4)
I believe the answer is:
trap
'the carriage' is the definition.
(trap is a kind of carriage)
'back section' is the wordplay.
'back' is a reversal indicator.
'section' becomes 'part' (synonyms).
'part' in reverse letter order is 'TRAP'.
'of' acts as a link.
(Other definitions for trap that I've seen before include "Means of transport - sometimes sprung!" , "mug" , "Driver's obstacle" , "that may catch a speeding motorist" , "Light carriage; ensnare" .)