ETs' ships (4)
I believe the answer is:
ufos
'ets' ships' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ufos that I've seen before include "Supposed spaceships" , "Mysterious phenomena" , "Initially, official flying saucers" , "Unauthorised traffic from outer space?" , "Mysterious objects in sky" .)