Lawman almost leaving car with boy (9)
I believe the answer is:
parkinson
'lawman' is the definition.
(C Northcote Parkinson devised 'Parkinson's law')
'almost leaving car with boy' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'leaving car' becomes 'parking'.
'with' is a charade indicator (letters next to each other).
'boy' becomes 'son' (synonyms).
'parking' with its last letter removed is 'parkin'.
'parkin'+'son'='PARKINSON'
(Other definitions for parkinson that I've seen before include "C. Northcote ---, writer, historian (---'s Law)" , "Man of the law" , "Lawman" , "[LAWMAKER]?" .)