Airborne trooper (4)
I believe the answer is:
para
'airborne trooper' is the definition.
(abbreviation of paratrooper)
This is all the clue.
(Other definitions for para that I've seen before include "Soldier that jumps from planes" , "soldier potentially dropped" , "Soldier who jumps to order? (abbr)" , "High-jumper" , "Soldier who's dropped in? (abbr)" .)