A vehicle crossing island of birds (5)
I believe the answer is:
avian
'of birds' is the definition.
(I've seen this before)
'a vehicle crossing island' is the wordplay.
'vehicle' becomes 'van' (van is a kind of vehicle).
'crossing' indicates putting letters inside.
'island' becomes 'i'.
'van' going around 'i' is 'vian'.
'a'+'vian'='AVIAN'
(Other definitions for avian that I've seen before include "Lizard with adhesive foot-pads" , "Birdlike" , "Like a booby" , "Regarding birds" , "Like rails, for example" .)