Alien craft, one commandeered by humans (7)
I believe the answer is:
martian
'alien' is the definition.
(a lifeform from Mars)
'craft one commandeered by humans' is the wordplay.
'craft' becomes 'art' (both can mean cunning or trickery).
'one' becomes 'I' (Roman numeral).
'commandeered by' indicates putting letters inside (commandeer can mean to take over).
'humans' becomes 'man' (man can mean humanity as a whole).
'art'+'i'='arti'
'arti' inserted into 'man' is 'MARTIAN'.
(Other definitions for martian that I've seen before include "Extraterrestrial" , "Green man from the Red Planet" , "One of H. G. Wells's invaders" , "Sci-fi character" , "Invader in The War of the Worlds (H. G. Wells)" .)