Closed down, broken by new crash (5)
I believe the answer is:
shunt
'crash' is the definition.
(I know that shunt is a type of crash)
'closed down broken by new' is the wordplay.
'closed down' becomes 'shut' (I am not sure about this - if you are sure you should believe this answer much more).
'broken by' means one lot of letters goes inside another (inserted letters break into the word).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'shut' enclosing 'n' is 'SHUNT'.
Can you help me to learn more?
(Other definitions for shunt that I've seen before include "Transfer rolling stock from track to track" , "Vehicle accident" , "Move rolling stock" , "Type of road accident" , "Car crash (slang)" .)