Vehicle beginning to cross a river (3)
I believe the answer is:
car
'vehicle' is the definition.
(I know that car is a type of wheeled vehicle)
'beginning to cross a river' is the wordplay.
'beginning to' indicates taking the first letters.
'river' becomes 'r' (abbreviation for river).
The initial letter of 'cross' is 'c'.
'c'+'a'+'r'='CAR'
(Other definitions for car that I've seen before include "4-wheeled vehicle" , "Wheeled vehicle" , "Four-wheeled vehicle" , "Motor vehicle" , "Hatchback, say?" .)