Testing old boy in parking lot (9)
I believe the answer is:
probation
'testing' is the definition.
(period of testing or assessment)
'old boy in parking lot' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'in' is an insertion indicator.
'parking' becomes 'p' (abbreviation on road signs).
'lot' becomes 'ration' (lot can mean a ration or allocation).
'p'+'ration'='pration'
'ob' put within 'pration' is 'PROBATION'.
(Other definitions for probation that I've seen before include "Test period of release from prison" , "Supervision of offenders" , "testing period" , "Trial period" , "Trial time for new worker" .)