Manic crashes involving one second-class vehicle (7)
I believe the answer is:
minicab
'vehicle' is the definition.
'manic crashes involving one second-class' is the wordplay.
'crashes' is an anagram indicator.
'involving' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'second class' becomes 'b'.
'manic' is an anagram of 'minca'.
'minca' placed around 'i' is 'minica'.
'minica'+'b'='MINICAB'
(Other definitions for minicab that I've seen before include "Metered vehicle for hire" , "Type of taxi" , "Car used as taxi" , "Type of car hire" , "It can't be hailed in the street" .)