By parking with hesitation (3)
I believe the answer is:
per
'by' is the definition.
(I've seen this in another clue)
'parking with hesitation' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'with' means one lot of letters go next to another.
'hesitation' becomes 'er' (sound used when hesitating).
'p'+'er'='PER'
(Other definitions for per that I've seen before include "For each or every" , "Preposition meaning for each" , "Preposition indicating by means of" , "According to" .)