A bill plugging public vehicles: you can count on them (8)
I believe the answer is:
abacuses
'you can count on them' is the definition.
(I've seen this in another clue)
'a bill plugging public vehicles' is the wordplay.
'bill' becomes 'ac' (short for account).
'plugging' is an insertion indicator (I've seen this in other clues).
'public vehicles' becomes 'buses' (I've seen this in another clue).
'ac' put within 'buses' is 'bacuses'.
'a'+'bacuses'='ABACUSES'
(Other definitions for abacuses that I've seen before include "they calculated" , "Counters" , "Count on these" , "old computing devices" , "Calculating devices" .)