Transport army team (4)
I believe the answer is:
taxi
'transport' is the definition.
(taxi is a kind of transport)
'army team' is the wordplay.
'army' becomes 'ta' (abbreviation for Territorial Army).
'team' becomes 'xi' (eleven, a football team).
'ta'+'xi'='TAXI'
(Other definitions for taxi that I've seen before include "Public service vehicle" , "Cab for hire" , "Car for hire" , "Rank occupant" , "Prepare to take off" .)