A commercial vehicle carrying one of birds (5)
I believe the answer is:
avian
'of birds' is the definition.
(I've seen this before)
'a commercial vehicle carrying one' is the wordplay.
'commercial vehicle' becomes 'van' (I've seen this before).
'carrying' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'van' enclosing 'i' is 'vian'.
'a'+'vian'='AVIAN'
(Other definitions for avian that I've seen before include "Like rails, for example" , "To do with birds - like a dangerous flu" , "Lizard with adhesive foot-pads" , "Birdlike" , "Like a booby" .)