Bunny heading to gaol in vehicle (4)
I believe the answer is:
bugs
'bunny' is the definition.
(I've seen this in another clue)
'heading to gaol in vehicle' is the wordplay.
'heading to' indicates taking the first letters (the head of the word).
'in' is an insertion indicator.
'vehicle' becomes 'bus' (public transport vehicle).
The initial letter of 'gaol' is 'g'.
'g' going inside 'bus' is 'BUGS'.
(Other definitions for bugs that I've seen before include "taps" , "Creepy-crawlies in computers" , "Small crawling creatures or disease-germs" , "Insects or faults in a system or machine" , "Insects or defects in computer programs" .)