American, one involved in transport of birds (5)
I believe the answer is:
avian
'of birds' is the definition.
(I've seen this before)
'american one involved in transport' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'one' becomes 'i' (Roman numeral).
'involved in' is an insertion indicator.
'transport' becomes 'van' (van is a kind of transport**).
'i' placed inside 'van' is 'vian'.
'a'+'vian'='AVIAN'
(Other definitions for avian that I've seen before include "Pertaining to or characteristic of birds" , "To do with birds - like a dangerous flu" , "Like a booby" , "Lizard with adhesive foot-pads" , "Resembling a bird" .)