Lever, prime requirement for the ship (7)
I believe the answer is:
trigger
'lever' is the definition.
(I know that trigger is a type of lever)
I don't understand the rest of the clue.
(Other definitions for trigger that I've seen before include "Lever to activate firing mechanism" , "Gun-firing device" , "Pull it to discharge gun" , "One pulled to fire" , "Part of gun" .)