One aunt travelling in the back of the vehicle (7)
I believe the answer is:
tonneau
'back of the vehicle' is the definition.
The definition suggests a singular noun which matches the answer.
'one aunt travelling' is the wordplay.
'travelling' indicates anagramming the letters.
'one'+'aunt'='oneaunt'
'oneaunt' anagrammed gives 'TONNEAU'.
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for tonneau that I've seen before include "car's back seat" , "Part of open car behind front seats" , "vintage car part" , "back of the car" , "cover for motorist" .)