Interplanetary craft ultimately needs steps with it (9)
I believe the answer is:
spaceship
'interplanetary craft' is the definition.
(I've seen this before)
'ultimately needs steps with it' is the wordplay.
'ultimately' suggests the final letters.
'steps' becomes 'paces' (pace is a kind of step).
'with it' becomes 'hip' (both can mean cool or fashionable).
The final letter of 'needs' is 's'.
's'+'paces'+'hip'='SPACESHIP'
(Other definitions for spaceship that I've seen before include "Craft to travel outside the Earth's atmosphere" , "long-distance traveller?" , "Interplanetary craft" , "long-distance vessel" , "craft going far?" .)