We can count on it if a vehicle reverses over us (6)
I believe the answer is:
abacus
'we can count on it' is the definition.
I can't tell whether this definition defines the answer.
'a vehicle reverses over us' is the wordplay.
'vehicle' becomes 'cab' (taxi cab).
'reverses' says the letters should be written in reverse.
'over' says to put letters next to each other (in a down clue, some letters go over others).
'cab' in reverse letter order is 'bac'.
'a'+'bac'+'us'='ABACUS'
'if' is the link.
Can you help me to learn more?
(Other definitions for abacus that I've seen before include "It will support the architrave" , "one counts on it" , "Counting device" , "not something we now count on" , "Early counting-frame" .)