Mechanic, if boarding lorry, starts to examine radiator (9)
I believe the answer is:
artificer
'mechanic' is the definition.
(I've seen this before)
'if boarding lorry starts to examine radiator' is the wordplay.
'boarding' is an insertion indicator.
'lorry' becomes 'artic' (articulated lorry is a semi-trailer truck).
'starts to' indicates taking the first letters.
The first letters of 'examine radiator' is 'er'.
'if' inserted within 'artic' is 'artific'.
'artific'+'er'='ARTIFICER'
(Other definitions for artificer that I've seen before include "Mechanic in the army or navy" , "One into skill" , "Military mechanic" , "Mechanic in the armed forces" , "Skilled worker" .)