Children behaving wildly in vehicle (7)
I believe the answer is:
chariot
'vehicle' is the definition.
(chariot is a kind of vehicle)
'children behaving wildly' is the wordplay.
'children' becomes 'ch' (genealogical abbreviation).
'behaving wildly' becomes 'ariot' (old-fashioned adverb).
'ch'+'ariot'='CHARIOT'
'in' acts as a link.
(Other definitions for chariot that I've seen before include "Ben Hur transport?" , "Horse-drawn racing vehicle; oat rich (anag.)" , "Roman two-wheeled vehicle" , "Ben Hur's car" , "Two-wheeled horse-drawn vehicle used in battle" .)