Knock around the railhead in a carriage (4)
I believe the answer is:
trap
'carriage' is the definition.
(I know that Trap is a type of carriage)
'knock around the railhead' is the wordplay.
'knock' becomes 'tap' (synonyms).
'around' is an insertion indicator.
'the railhead' becomes 'r' (head letter of 'rail').
'tap' going around 'r' is 'TRAP'.
'in a' is the link.
(Other definitions for trap that I've seen before include "The Web" , "Baited device" , "that can let an actor down" , "Lure, snare" , "Bunker; carriage" .)