What must be cracked on vehicle (5)
I believe the answer is:
rebus
'what must be cracked' is the definition.
(a rebus is a visual puzzle to be cracked)
'on vehicle' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'vehicle' becomes 'bus' (bus is a kind of vehicle).
're'+'bus'='REBUS'
(Other definitions for rebus that I've seen before include "Puzzling device" , "Word-picture puzzle" , "Pictorial teaser" , "Word puzzle involving pictures" , "Enigmatic representation" .)