Arrest zero students occupying vehicle (6)
I believe the answer is:
collar
'arrest' is the definition.
(synonyms)
'zero students occupying vehicle' is the wordplay.
'zero' becomes 'O' (O resembles 0).
'students' becomes 'LL' (two abbreviations for learner).
'occupying' indicates putting letters inside (inserted letters occupy or enter the word).
'vehicle' becomes 'car' (car is a kind of vehicle).
'o'+'ll'='oll'
'oll' placed within 'car' is 'COLLAR'.
(Other definitions for collar that I've seen before include "Ruff" , "Neckband to seize" , "Appropriate - piece of neckwear" , "One with "dog" identifies a clergyman" , "Apprehend (colloq.); cut of bacon" .)