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 judge whether this definition defines the answer.
'a vehicle reverses over us' is the wordplay.
'vehicle' becomes 'cab' (cab is a kind of vehicle).
'reverses' is a reversal indicator.
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'cab' back-to-front is 'bac'.
'a'+'bac'+'us'='ABACUS'
'if' acts as a 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" .)