Vehicle parked in picnic area (3)
I believe the answer is:
car
'vehicle' is the definition.
(car is a kind of vehicle)
'parked in picnic area' is the wordplay.
'parked in' says the answer is hidden in the clue.
'CAR' is hidden within 'picnic area'.
(Other definitions for car that I've seen before include "Private transport" , "estate, perhaps" , "Vehicle carrying vehicles" , "Means of transport" , "Railway carriage; vehicle" .)