Transport add-on, one given to appropriate vehicles? (8)
I believe the answer is:
joyrider
'one given to appropriate vehicles?' is the definition.
(person who races in stolen vehicles)
'transport add-on' is the wordplay.
'transport' becomes 'joy' (both can mean bliss or ecstasy).
'add-on' becomes 'rider' (an add-on in a contract).
'joy'+'rider'='JOYRIDER'
(Other definitions for joyrider that I've seen before include "Person who drives around in stolen cars" , "He drives stolen cars for fun - sounds ecstatic" , "Criminal driver" , "Driver of car stolen for fun" , "Driver of stolen vehicle" .)